Potentially breaking changed in URL building

[jlcsilva]

This PR introduced potentially breaking changes in the way URLs are built.

Previously, the input url=http://example.com?param=test and params={} would result in a final self.url=http://example.com?param=test.

Now, the same input results in self.url=http://example.com, completely removing the query parameters.

[zanieb]

This looks like a duplicate of #3433

[jlcsilva]

Oh sorry. I tried to look it up under the api breaking changes, but only open ones.

Since the original has been closed, feel free to close this too.

[tomchristie]

Thanks @jlcsilva also an issue on our side, specifically that https://github.com/encode/httpx/blob/master/CHANGELOG.md doesn't mention this change. If someone's motivated to add a clean concise entry that'd be welcome.