Types: Fix type annotation for default_encoding parameter

[JelleZijlstra]

The default encoding function is allowed to return None, in which case
the encoding property of httpx.Response defaults to utf-8.

The example in https://www.python-httpx.org/advanced/#using-character-set-auto-detection
that passes a default_encoding function uses a function that may return None.

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.