Transform the mime object to symbol when registering the parsers

This will keep our current API working without having the users to
change their codebases.

actionpack/lib/action_dispatch/http/parameters.rb

@@ -1,6 +1,8 @@
 module ActionDispatch
   module Http
     module Parameters
+      extend ActiveSupport::Concern
+
       PARAMETERS_KEY = 'action_dispatch.request.path_parameters'
 
       DEFAULT_PARSERS = {
@@ -10,13 +12,20 @@ module Parameters
         }
       }
 
-      def self.included(klass)
-        class << klass
-          attr_accessor :parameter_parsers
+      included do
+        class << self
+          attr_reader :parameter_parsers
         end
 
-        klass.parameter_parsers = DEFAULT_PARSERS
+        self.parameter_parsers = DEFAULT_PARSERS
       end
+
+      module ClassMethods
+        def parameter_parsers=(parsers) # :nodoc:
+          @parameter_parsers = parsers.transform_keys { |key| key.respond_to?(:symbol) ? key.symbol : key }
+        end
+      end
+
       # Returns both GET and POST \parameters in a single hash.
       def parameters
         params = get_header("action_dispatch.request.parameters")

actionpack/lib/action_dispatch/middleware/params_parser.rb

@@ -37,6 +37,7 @@ def original_exception
     # 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 self.new(app, parsers = {})
+      parsers = parsers.transform_keys { |key| key.respond_to?(:symbol) ? key.symbol : key }
       ActionDispatch::Request.parameter_parsers = ActionDispatch::Request::DEFAULT_PARSERS.merge(parsers)
       app
     end

actionpack/test/controller/webservice_test.rb

@@ -65,7 +65,7 @@ def test_put_json
 
   def test_register_and_use_json_simple
     with_test_route_set do
-      with_params_parsers json: Proc.new { |data| ActiveSupport::JSON.decode(data)['request'].with_indifferent_access } do
+      with_params_parsers Mime[:json] => Proc.new { |data| ActiveSupport::JSON.decode(data)['request'].with_indifferent_access } do
         post "/",
           params: '{"request":{"summary":"content...","title":"JSON"}}',
           headers: { 'CONTENT_TYPE' => 'application/json' }