Merge a bunch of old PRs into one, add content-encoding (compression) support.

[rambo]

Did a bit of cherry-picking.

Includes and supercedes:

• https://github.com/slank/awsgi/pull/63
• https://github.com/slank/awsgi/pull/60
• https://github.com/slank/awsgi/pull/58
• https://github.com/slank/awsgi/pull/56

Additionally fixes https://github.com/slank/awsgi/issues/19

[rambo]

I also plan to completely modernize this library but that will be a separate package called awsgi2, I'll be posting links when it's time.

[symroe]

FWIW I've mostly moved to https://mangum.io/ since opening the above PRs

[rambo]

https://gitlab.com/advian-oss/python-awsgi2 for the fork for people still stuck with non async WSGI apps.