bugfix for zstd decompressobj reuse

[mbeijen]

Fixes https://github.com/encode/httpx/discussions/3538

Summary

This fixes an issue where zstd decoding fails. I've included a test case.

Checklist

• [ x ] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [ x ] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [ x ] I've updated the documentation accordingly.