Fix params replacing query set in URL

[KeeganOP]

Summary

Fixes #905 and resolves comments raised on PR: #961 by pulling over https://codeberg.org/httpxyz/httpxyz/commit/417d6c6fa193c5f29cdacefef6a4f7c9e5d30313 from @mbeijen

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.

[codspeed-hq]

Merging this PR will not alter performance

✅ 7 untouched benchmarks

---

_{Comparing KeeganOP:params-replacing (3f77839) with main (c1d3d95)}

[KeeganOP]

I spotted this while testing as well:

client = httpx2.Client(base_url="https://api.example.com/v1?base_url=true", params={"api_key": "abc"})
request = client.build_request("GET", "users?active=true", params={"page": "2"})

assert str(request.url) == "https://api.example.com/v1/users?base_url=true&active=true&api_key=abc&page=2"

fails since the request.url comes out as:

https://api.example.com/v1?base_url=true/users?active=true&api_key=abc&page=2

Expected behavior? If not I'll update this PR to handle the above