Modify unknown default to None for json, form and json_or_form

[lafrech]

Closes #580.

No test was broken and I didn't add any.

CHANGELOG will need to be sorted out before releasing but I did it this way to minimize merge conflicts.

[sirosen]

I'm not sure I quite understand the mypy failure.

It looks like it's failing in cases where we've subclassed and override DEFAULT_UNKNOWN_BY_LOCATION because it deduces the type as Mapping[str, str] and then we include **Parser.DEFAULT_UNKNOWN_BY_LOCATION which now is a subtype of Mapping[str, Optional[str]]...?

Maybe the solution is just to explicitly add a type annotation in the pyramid parser and the aiohttp parser.

The change itself looks good -- exactly what we talked about and you put forth as a better default -- we just need to get the linting to agree that it's good. 😉

[lafrech]

Yeah, I don't get it either. I suspect a mypy issue but I'm not educated enough to even file a bug report.

Maybe the solution is just to explicitly add a type annotation in the pyramid parser and the aiohttp parser.

It's late so I won't investigate any further now. I just did this and it passed.

[sirosen]

Looks good! 👍