fix Message.to_dict mutating the underlying Message (#378)

* [fix] to_dict modifies the underlying message (#151)

* add test for mapmessage

* fix for to_dict

* formatting

* Apply suggestions from code review

Co-authored-by: Arun Babu Neelicattu <arun.neelicattu@gmail.com>

* change to_json to to_dict

Co-authored-by: Arun Babu Neelicattu <arun.neelicattu@gmail.com>