Switch environ usage to headers in request mixins

[pgjones]

This allows these mixins to be used in ASGI frameworks that have no
concept of environs but do have headers.

The downside is the extra indirection, although the headers are a cached property and the environ/headers are immutable.

[davidism]

Had to add # type: ignore to the mixins, luckily there weren't a lot of uses. I've been thinking about this for a while, but I really think the mixins should inherit from BaseRequest. Or they should mostly be integrated into BaseRequest or Request since they don't add any overhead, just more attributes and methods.

[pgjones]

Thanks 🎉