Add `as` to encode a request as a specific mime type.

Turns

```
post articles_path(format: :json), params: { article: { name: 'Ahoy!' } }.to_json,
  headers: { 'Content-Type' => 'application/json' }
```

into

```
post articles_path, params: { article: { name: 'Ahoy!' } }, as: :json
```

actionpack/lib/action_dispatch/testing/integration.rb

@@ -321,7 +321,9 @@ def non_kwarg_request_warning
         end
 
         # Performs the actual request.
-        def process(method, path, params: nil, headers: nil, env: nil, xhr: false)
+        def process(method, path, params: nil, headers: nil, env: nil, xhr: false, as: nil)
+          request_encoder = RequestEncoder.encoder(as)
+
           if path =~ %r{://}
             location = URI.parse(path)
             https! URI::HTTPS === location if location.scheme
@@ -330,14 +332,17 @@ def process(method, path, params: nil, headers: nil, env: nil, xhr: false)
               url_host += ":#{location.port}" if default != location.port
               host! url_host
             end
-            path = location.query ? "#{location.path}?#{location.query}" : location.path
+            path = request_encoder.append_format_to location.path
+            path = location.query ? "#{path}?#{location.query}" : path
+          else
+            path = request_encoder.append_format_to path
           end
 
           hostname, port = host.split(':')
 
           request_env = {
             :method => method,
-            :params => params,
+            :params => request_encoder.encode_params(params),
 
             "SERVER_NAME"     => hostname,
             "SERVER_PORT"     => port || (https? ? "443" : "80"),
@@ -347,7 +352,7 @@ def process(method, path, params: nil, headers: nil, env: nil, xhr: false)
             "REQUEST_URI"    => path,
             "HTTP_HOST"      => host,
             "REMOTE_ADDR"    => remote_addr,
-            "CONTENT_TYPE"   => "application/x-www-form-urlencoded",
+            "CONTENT_TYPE"   => request_encoder.content_type,
             "HTTP_ACCEPT"    => accept
           }
 
@@ -387,6 +392,48 @@ def process(method, path, params: nil, headers: nil, env: nil, xhr: false)
         def build_full_uri(path, env)
           "#{env['rack.url_scheme']}://#{env['SERVER_NAME']}:#{env['SERVER_PORT']}#{path}"
         end
+
+        class RequestEncoder # :nodoc:
+          @encoders = {}
+
+          def initialize(mime_name, param_encoder, url_encoded_form = false)
+            @mime = Mime[mime_name]
+
+            unless @mime
+              raise ArgumentError, "Can't register a request encoder for " \
+                "unregistered MIME Type: #{mime_name}. See `Mime::Type.register`."
+            end
+
+            @url_encoded_form = url_encoded_form
+            @path_format      = ".#{@mime.symbol}" unless @url_encoded_form
+            @param_encoder    = param_encoder || :"to_#{@mime.symbol}".to_proc
+          end
+
+          def append_format_to(path)
+            path << @path_format unless @url_encoded_form
+            path
+          end
+
+          def content_type
+            @mime.to_s
+          end
+
+          def encode_params(params)
+            @param_encoder.call(params)
+          end
+
+          def self.encoder(name)
+            @encoders[name] || WWWFormEncoder
+          end
+
+          def self.register_encoder(mime_name, &param_encoder)
+            @encoders[mime_name] = new(mime_name, param_encoder)
+          end
+
+          register_encoder :json
+
+          WWWFormEncoder = new(:url_encoded_form, -> params { params }, true)
+        end
     end
 
     module Runner
@@ -643,6 +690,30 @@ def method_missing(sym, *args, &block)
   #       end
   #   end
   #
+  # You can also test your JSON API easily by setting what the request should
+  # be encoded as:
+  #
+  #   require 'test_helper'
+  #
+  #   class ApiTest < ActionDispatch::IntegrationTest
+  #     test "creates articles" do
+  #       assert_difference -> { Article.count } do
+  #         post articles_path, params: { article: { title: 'Ahoy!' } }, as: :json
+  #       end
+  #
+  #       assert_response :success
+  #     end
+  #   end
+  #
+  # The `as` option sets the format to JSON, sets the content type to
+  # 'application/json' and encodes the parameters as JSON.
+  #
+  # For any custom MIME Types you've registered, you can even add your own encoders with:
+  #
+  #   ActionDispatch::IntegrationTest.register_encoder :wibble do |params|
+  #     params.to_wibble
+  #   end
+  #
   # Consult the Rails Testing Guide for more.
 
   class IntegrationTest < ActiveSupport::TestCase
@@ -671,5 +742,9 @@ def url_options
     def document_root_element
       html_document.root
     end
+
+    def self.register_encoder(*args, &param_encoder)
+      Integration::Session::RequestEncoder.register_encoder(*args, &param_encoder)
+    end
   end
 end

actionpack/test/controller/integration_test.rb

@@ -1126,3 +1126,46 @@ def test_cookies_set_in_setup_are_persisted_through_the_session
     assert_equal({"user_name"=>"david"}, cookies.to_hash)
   end
 end
+
+class IntegrationRequestEncodersTest < ActionDispatch::IntegrationTest
+  class FooController < ActionController::Base
+    def foos
+      render plain: 'ok'
+    end
+  end
+
+  def test_encoding_as_json
+    assert_encoded_as :json, content_type: 'application/json'
+  end
+
+  def test_encoding_as_without_mime_registration
+    assert_raise ArgumentError do
+      ActionDispatch::IntegrationTest.register_encoder :wibble
+    end
+  end
+
+  def test_registering_custom_encoder
+    Mime::Type.register 'text/wibble', :wibble
+
+    ActionDispatch::IntegrationTest.register_encoder(:wibble, &:itself)
+
+    assert_encoded_as :wibble, content_type: 'text/wibble',
+      parsed_parameters: Hash.new # Unregistered MIME Type can't be parsed
+  ensure
+    Mime::Type.unregister :wibble
+  end
+
+  private
+    def assert_encoded_as(format, content_type:, parsed_parameters: { 'foo' => 'fighters' })
+      with_routing do |routes|
+        routes.draw { post ':action' => FooController }
+
+        post '/foos', params: { foo: 'fighters' }, as: format
+
+        assert_response :success
+        assert_match "foos.#{format}", request.path
+        assert_equal content_type, request.content_type
+        assert_equal parsed_parameters, request.request_parameters
+      end
+    end
+end