diff --git a/http.gemspec b/http.gemspec
index 7cabad52..2c2c79b4 100644
--- a/http.gemspec
+++ b/http.gemspec
@@ -24,6 +24,7 @@ Gem::Specification.new do |gem|
   gem.version       = HTTP::VERSION
 
   gem.add_runtime_dependency 'http_parser.rb', '~> 0.6.0'
+  gem.add_runtime_dependency 'form_data',      '~> 0.0.1'
 
   gem.add_development_dependency 'bundler', '~> 1.0'
 end
diff --git a/lib/http/client.rb b/lib/http/client.rb
index 7c9a7740..1390ceda 100644
--- a/lib/http/client.rb
+++ b/lib/http/client.rb
@@ -1,5 +1,6 @@
 require 'cgi'
 require 'uri'
+require 'form_data'
 require 'http/options'
 require 'http/redirector'
 
@@ -127,12 +128,15 @@ def normalize_uri(uri)
 
     # Create the request body object to send
     def make_request_body(opts, headers)
-      if opts.body
+      case
+      when opts.body
         opts.body
-      elsif opts.form
-        headers['Content-Type'] ||= 'application/x-www-form-urlencoded'
-        URI.encode_www_form(opts.form)
-      elsif opts.json
+      when opts.form
+        form = FormData.create opts.form
+        headers['Content-Type']   ||= form.content_type
+        headers['Content-Length'] ||= form.content_length
+        form.to_s
+      when opts.json
         headers['Content-Type'] ||= 'application/json'
         MimeType[:json].encode opts.json
       end
diff --git a/lib/http/request/writer.rb b/lib/http/request/writer.rb
index f4044ce9..7586cf3a 100644
--- a/lib/http/request/writer.rb
+++ b/lib/http/request/writer.rb
@@ -34,13 +34,8 @@ def stream
       def add_body_type_headers
         if @body.is_a?(String) && !@headers['Content-Length']
           @request_header << "Content-Length: #{@body.bytesize}"
-        elsif @body.is_a?(Enumerable)
-          encoding = @headers['Transfer-Encoding']
-          if encoding == 'chunked'
-            @request_header << 'Transfer-Encoding: chunked'
-          else
-            fail(RequestError, 'invalid transfer encoding')
-          end
+        elsif @body.is_a?(Enumerable) && 'chunked' != @headers['Transfer-Encoding']
+          fail(RequestError, 'invalid transfer encoding')
         end
       end
 
diff --git a/spec/http/request/writer_spec.rb b/spec/http/request/writer_spec.rb
index 0481c078..b948b6df 100644
--- a/spec/http/request/writer_spec.rb
+++ b/spec/http/request/writer_spec.rb
@@ -3,41 +3,89 @@
 require 'spec_helper'
 
 RSpec.describe HTTP::Request::Writer do
+  let(:io)          { StringIO.new }
+  let(:body)        { '' }
+  let(:headers)     { HTTP::Headers.new }
+  let(:headerstart) { 'GET /test HTTP/1.1' }
+
+  subject(:writer)  { described_class.new(io, body, headers, headerstart) }
+
   describe '#initalize' do
-    def construct(body)
-      HTTP::Request::Writer.new(nil, body, [], '')
-    end
+    context 'when body is nil' do
+      let(:body) { nil }
 
-    it "doesn't throw on a nil body" do
-      expect { construct nil }.not_to raise_error
+      it 'does not raise an error' do
+        expect { writer }.not_to raise_error
+      end
     end
 
-    it "doesn't throw on a String body" do
-      expect { construct 'string body' }.not_to raise_error
-    end
+    context 'when body is a string' do
+      let(:body) { 'string body' }
 
-    it "doesn't throw on an Enumerable body" do
-      expect { construct %w(bees cows) }.not_to raise_error
+      it 'does not raise an error' do
+        expect { writer }.not_to raise_error
+      end
     end
 
-    it "does throw on a body that isn't string, enumerable or nil" do
-      expect { construct true }.to raise_error
+    context 'when body is an Enumerable' do
+      let(:body) { %w(bees cows) }
+
+      it 'does not raise an error' do
+        expect { writer }.not_to raise_error
+      end
     end
 
-    it 'writes a chunked request from an Enumerable correctly' do
-      io = StringIO.new
-      writer = HTTP::Request::Writer.new(io, %w(bees cows), [], '')
-      writer.send_request_body
-      io.rewind
-      expect(io.string).to eq "4\r\nbees\r\n4\r\ncows\r\n0\r\n\r\n"
+    context 'when body is not string, enumerable or nil' do
+      let(:body) { 123 }
+
+      it 'raises an error' do
+        expect { writer }.to raise_error
+      end
     end
   end
 
-  describe '#add_body_type_headers' do
-    it 'properly calculates length of unicode string' do
-      writer = HTTP::Request::Writer.new(nil, 'Привет, мир!', {}, '')
-      writer.add_body_type_headers
-      expect(writer.join_headers).to match(/\r\nContent-Length: 21\r\n/)
+  describe '#stream' do
+    context 'when body is Enumerable' do
+      let(:body)    { %w(bees cows) }
+      let(:headers) { HTTP::Headers.coerce 'Transfer-Encoding' => 'chunked' }
+
+      it 'writes a chunked request from an Enumerable correctly' do
+        writer.stream
+        expect(io.string).to end_with "\r\n4\r\nbees\r\n4\r\ncows\r\n0\r\n\r\n"
+      end
+
+      it 'writes Transfer-Encoding header only once' do
+        writer.stream
+        expect(io.string).to start_with "#{headerstart}\r\nTransfer-Encoding: chunked\r\n\r\n"
+      end
+
+      context 'when Transfer-Encoding not set' do
+        let(:headers) { HTTP::Headers.new }
+        specify { expect { writer.stream }.to raise_error }
+      end
+
+      context 'when Transfer-Encoding is not chunked' do
+        let(:headers) { HTTP::Headers.coerce 'Transfer-Encoding' => 'gzip' }
+        specify { expect { writer.stream }.to raise_error }
+      end
+    end
+
+    context 'when body is a unicode String' do
+      let(:body) { 'Привет, мир!' }
+
+      it 'properly calculates Content-Length if needed' do
+        writer.stream
+        expect(io.string).to start_with "#{headerstart}\r\nContent-Length: 21\r\n\r\n"
+      end
+
+      context 'when Content-Length explicitly set' do
+        let(:headers) { HTTP::Headers.coerce 'Content-Length' => 12 }
+
+        it 'keeps given value' do
+          writer.stream
+          expect(io.string).to start_with "#{headerstart}\r\nContent-Length: 12\r\n\r\n"
+        end
+      end
     end
   end
 end