Fields filtering with include #1475

# JSONAPI::ResourceSerializer#supplying_relationship_fields

def supplying_relationship_fields(resource_klass)
        @_supplying_relationship_fields.fetch resource_klass do
          relationships = Set.new(resource_klass._relationships.keys.map(&:to_sym))
          cur = resource_klass
          while cur != JSONAPI::Resource
            if @fields.has_key?(cur._type)
              # here... 
              # It always empty the Set
              #relationships &= @fields[cur._type]
              break
            end
            cur = cur.superclass
          end
          @_supplying_relationship_fields[resource_klass] = relationships
        end
      end