Improve  _objectKeys method
Improve  _objectKeys method

https://github.com/Starcounter-Jack/JSON-Patch/issues/85

Starcounter-Jack/JSON-Patch,Starcounter-Jack/JSON-Patch,Starcounter-Jack/JSON-Patch,Starcounter-Jack/JSON-Patch,baranga/JSON-Patch,baranga/JSON-Patch,baranga/JSON-Patch,baranga/JSON-Patch