fix: route.continue should not change multipart form data body

spec/integration/page/request_continue_spec.rb

@@ -0,0 +1,40 @@
+require 'spec_helper'
+
+RSpec.describe 'request#continue' do
+  it 'should work', sinatra: true do
+    with_page do |page|
+      page.route('**/*', -> (route, _) { route.continue })
+      page.goto(server_empty_page)
+    end
+  end
+
+  it 'continue should not change multipart/form-data body', sinatra: true do
+    with_page do |page|
+      page.goto(server_empty_page)
+      promise = Concurrent::Promises.resolvable_future
+      sinatra.post('/upload') do
+        content_type 'text/plain'
+        promise.fulfill(params[:file][:tempfile].read)
+        'done'
+      end
+
+      page.route('**/*', -> (route, _) { route.continue })
+
+      status = page.evaluate(<<~JAVASCRIPT)
+      async () => {
+        const newFile = new File(['file content'], 'file.txt');
+        const formData = new FormData();
+        formData.append('file', newFile);
+        const response = await fetch('/upload', {
+          method: 'POST',
+          credentials: 'include',
+          body: formData,
+        });
+        return response.status;
+      }
+      JAVASCRIPT
+      expect(status).to eq(200)
+      expect(promise.value!).to eq('file content')
+    end
+  end
+end