fix(decoder): decode multi-member gzip data

[lizeyan]

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Summary

https://github.com/encode/httpx/discussions/3269

Allow GZipDecoder to decode multi-member gzip data

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.