Allow to define custom serializer for given class by defining #serializer_class method in serialized object's class. Resolves #515.

test/serializers/serializer_for_test.rb

@@ -29,10 +29,14 @@ module ActiveModel
       class SerializerTest <  Minitest::Test
         class MyProfile < Profile
         end
+        class CustomProfile
+          def serializer_class; ProfileSerializer; end
+        end
 
         def setup
           @profile = Profile.new
           @my_profile = MyProfile.new
+          @custom_profile = CustomProfile.new
           @model = ::Model.new
         end
 
@@ -50,6 +54,11 @@ module ActiveModel
           serializer = ActiveModel::Serializer.serializer_for(@my_profile)
           assert_equal ProfileSerializer, serializer
         end
+
+        def test_serializer_custom_serializer
+          serializer = ActiveModel::Serializer.serializer_for(@custom_profile)
+          assert_equal ProfileSerializer, serializer
+        end
       end
     end
   end