Add symbol support for ActiveModel::Serializer.type method

The ActiveModel::Serializer.type method now accepts symbol as paremeter:
class AuthorSerializer < ActiveModel::Serializer
  type :profile
end
The test file for the type was also refactored.

lib/active_model/serializer/type.rb

@@ -17,7 +17,7 @@ module ActiveModel
         #   class AdminAuthorSerializer < ActiveModel::Serializer
         #     type 'authors'
         def type(type)
-          self._type = type
+          self._type = type && type.to_s
         end
       end
     end