Add content_type for Response

[Sanchoyzer]

Summary

I would like to suggest adding a little syntax sugar. It might be more convenient to use response.content_type instead of response.headers.get("Content-Type").
I hope I haven't forgotten any places where changes need to be made

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.

[Sanchoyzer]

Hey, @tomchristie , what do you think of this suggestion?

[Kludex]

Why the str() cast? Isn't it str already?

[Sanchoyzer]

It's not str, it's Any

class Headers(typing.MutableMapping[str, str]):
    def get(self, key: str, default: typing.Any = None) -> typing.Any:
        ...

[Kludex]

I think this is not needed, but we can see what other maintainers think about it.