Allow Urlencoded to be instanciated with an encoder.

lib/http/form_data.rb

@@ -43,9 +43,15 @@ class << self
       # @return [Urlencoded] otherwise
       def create(data)
         data  = ensure_hash data
-        klass = multipart?(data) ? Multipart : Urlencoded
+        if multipart?(data)
+          Multipart.new data
+        else
+          Urlencoded.new data, encoder
+        end
+      end
 
-        klass.new data
+      def encoder
+        ::URI.method(:encode_www_form)
       end
 
       # Coerce `obj` to Hash.

lib/http/form_data/urlencoded.rb

@@ -12,8 +12,8 @@ class Urlencoded
       include Readable
 
       # @param [#to_h, Hash] data form data key-value Hash
-      def initialize(data)
-        encoded_data = encode(data)
+      def initialize(data, encoder = ::URI.method(:encode_www_form))
+        encoded_data = encoder.call(FormData.ensure_hash(data))
         @io = StringIO.new(encoded_data)
       end
 
@@ -29,11 +29,6 @@ def content_type
       #
       # @return [Integer]
       alias content_length size
-
-      # @param [#to_h, Hash] data form data key-value Hash
-      def encode(data)
-        ::URI.encode_www_form(FormData.ensure_hash(data))
-      end
     end
   end
 end