fix: merge existing params in url, close #3433

[GreyElaina]

Summary

Revert #3364

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.

Edit by @Kludex :

• Closeshttps://github.com/encode/httpx/issues/3433

[Kludex]

Can we add a test here, so we are sure this doesn't break again in the future?

We need a test.

[GreyElaina]

Can we add a test here, so we are sure this doesn't break again in the future?

no problem, will present later.

[GreyElaina]

@Kludex tests are ready.

[lovelydinosaur]

Thanks, tho see https://github.com/encode/httpx/issues/3433#issuecomment-2514327255.

My suggestion would be...

• Introduce tests for the 0.28.0 behaviour.
• Include a release note item in 0.28.0 documenting the behavior change.

[GreyElaina]

Thanks, tho see #3433 (comment).

My suggestion would be...

* Introduce tests for the 0.28.0 behaviour.
* Include a release note item in 0.28.0 documenting the behavior change.

fine, see you another pr
close is welcome.