Allow to define custom serializer for given class by defining #serial…

…izer_class method in serialized object's class. Resolves rails-api#515.

lib/active_model/serializer.rb

@@ -115,7 +115,9 @@ def self.urls(*attrs)
     end
 
     def self.serializer_for(resource, options = {})
-      if resource.respond_to?(:to_ary)
+      if resource.respond_to?(:serializer_class)
+        resource.serializer_class
+      elsif resource.respond_to?(:to_ary)
         config.array_serializer
       else
         options

test/serializers/serializer_for_test.rb

@@ -29,10 +29,14 @@ def test_overwritten_serializer_for_array
       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 @@ def test_serializer_inherited_serializer
           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