Fixes #2863

Summary

Since the presense or absense of the Accept-Encoding header can change the response body, Vary should always be added whether the client requested gzip encoding or not. Otherwise some caching implementations might return a non-compressed response even when a compressed one was requested.

This change may seem a bit complex, but it reworks the GZipMiddleware to behave the same no matter what the compression is (or lack of). Basic changes include:

1. Move all gzip compression to a method apply_compression and don't update the content-encoding or content-length headers if the body didn't change.
2. Create a base class IdentityResponder from GZipResponder which will be used to respond to non-gzip requests. Its apply_compression method will return the body unchanged.
3. Coincidentally, it's now easier to implement new compression algorithms like deflate, brotli, or zstd. This PR does not implement those.

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.