5113. Validation of oneof nested objects (#5211)

* Adds instruction for test codegen

* Adds tests to reproduce issue 5113. Oneof object should not override other oneof fields.

* Renames a new examplepb field for еру sake of clarity

* Checks if a oneOf field is already set before the call of msgValue.Mutable, that resets other options of the oneOf.

* Reverts Contributing.md

* Regenerates swagger

Author: optician