feat(proxy_headers): add support for x-forwarded-host

[aberlioz]

Summary

Fixes #965

Add support for X-Forwarded-Host header in uvicorn.

If a non-empty value is found for X-Forwarded-Host:

• Updates scope["server"] with (host, port) values
• Updates value of host header in scope["headers"]

Note

This is my first contribution to uvicorn, feedback is welcomed 🙏🏻

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.