Support JSON serialization options for model objects (#811)

lib/google/apis/core/json_representation.rb

@@ -145,7 +145,7 @@ module Google
 
         def to_json(*a)
           representation = self.class.const_get(:Representation)
-          representation.new(self).to_json(user_options: { skip_undefined: true })
+          representation.new(self).to_hash(user_options: { skip_undefined: true }).to_json(*a)
         end
       end
     end