do not instantiate a param parser middleware

we don't actually need a param parser middleware instance since the
request object will take care of parsing parameters for us.  For now,
we'll just configure the parameter parsers on the request in this class.

actionpack/lib/action_dispatch/middleware/params_parser.rb

@@ -22,13 +22,9 @@ module ActionDispatch
     #
     # The +parsers+ argument can take Hash of parsers where key is identifying
     # content mime type, and value is a lambda that is going to process data.
-    def initialize(app, parsers = {})
-      @app = app
+    def self.new(app, parsers = {})
       ActionDispatch::Request.parameter_parsers = ActionDispatch::Request::DEFAULT_PARSERS.merge(parsers)
-    end
-
-    def call(env)
-      @app.call(env)
+      app
     end
   end
 end

actionpack/test/controller/webservice_test.rb

@@ -97,24 +97,28 @@ class WebServiceTest < ActionDispatch::IntegrationTest
   end
 
   def test_parsing_json_doesnot_rescue_exception
-    with_test_route_set do
-      with_params_parsers Mime::JSON => Proc.new { |data| raise Interrupt } do
-        assert_raises(Interrupt) do
-          post "/",
-            params: '{"title":"JSON"}}',
-            headers: { 'CONTENT_TYPE' => 'application/json' }
-        end
+    req = Class.new(ActionDispatch::Request) do
+      def params_parsers
+        { Mime::JSON => Proc.new { |data| raise Interrupt } }
       end
+
+      def content_length; get_header('rack.input').length; end
+    end.new({ 'rack.input' => StringIO.new('{"title":"JSON"}}'), 'CONTENT_TYPE' => 'application/json' })
+
+    assert_raises(Interrupt) do
+      req.request_parameters
     end
   end
 
   private
     def with_params_parsers(parsers = {})
       old_session = @integration_session
-      @app = ActionDispatch::ParamsParser.new(app.routes, parsers)
+      original_parsers = ActionDispatch::Request.parameter_parsers
+      ActionDispatch::Request.parameter_parsers = original_parsers.merge parsers
       reset!
       yield
     ensure
+      ActionDispatch::Request.parameter_parsers = original_parsers
       @integration_session = old_session
     end