Change JSONResponse headers type annotation

[connesy]

Summary

As agreed in this discussion (#2186), the type annotation for the headers parameter of JSONResponse is changed from typing.Optional[typing.Dict[str, str]] to typing.Optional[typing.Mapping[str, str]] to align with the other response classes.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.