Fix URL params to merge with existing query parameters instead of replacing them

[ferstar]

Summary

This PR fixes a long-standing issue where URL(url, params=params) and Request(method, url, params=params) would completely replace existing query parameters in the URL instead of merging them. This behavior was inconsistent with the Python requests library and unintuitive for users.

Problem

Previously, when creating a URL or Request with additional parameters:

# Before this fix
url = httpx.URL("https://api.example.com?token=abc123", params={"page": "1"})
print(url)  # https://api.example.com?page=1  (token=abc123 was lost!)

request = httpx.Request("GET", "https://api.example.com?token=abc123", params={"page": "1"})
print(request.url)  # https://api.example.com?page=1  (token=abc123 was lost!)

The original query parameters (token=abc123) were completely discarded.

Solution

After this fix, parameters are intelligently merged:

# After this fix
url = httpx.URL("https://api.example.com?token=abc123", params={"page": "1"})
print(url)  # https://api.example.com?token=abc123&page=1  (parameters merged!)

request = httpx.Request("GET", "https://api.example.com?token=abc123", params={"page": "1"})
print(request.url)  # https://api.example.com?token=abc123&page=1  (parameters merged!)

Behavior Details

The new logic handles various cases intelligently:

1. Non-empty params: Merged with existing query parameters

   URL("https://example.com?a=1", params={"b": "2"})
   # Result: "https://example.com?a=1&b=2"

2. Empty dict params: Preserves existing query parameters

   URL("https://example.com?a=1", params={})
   # Result: "https://example.com?a=1"

3. None params: Preserves existing query parameters

   URL("https://example.com?a=1", params=None)
   # Result: "https://example.com?a=1"

4. Overlapping parameter names: New values override old ones

   URL("https://example.com?a=old", params={"a": "new", "b": "2"})
   # Result: "https://example.com?a=new&b=2"

5. QueryParams objects: Used directly (preserves existing copy_* method behavior)

   url = URL("https://example.com?a=1&b=2")
   url.copy_remove_param("a")  # Still works as expected
   # Result: "https://example.com?b=2"

Compatibility

• ✅ Backwards compatible: All existing copy_* methods work unchanged
• ✅ Test coverage: All existing tests pass with updated expectations
• ✅ Consistent with requests: Matches Python requests library behavior
• ✅ Intuitive: Behavior now matches user expectations

Files Changed

• httpx/_urls.py: Updated URL.init params handling logic
• tests/models/test_url.py: Updated test expectations for merge behavior
• tests/models/test_requests.py: Updated test expectations for merge behavior

Testing

Added comprehensive tests covering:

• Basic parameter merging
• Edge cases (empty params, None params, overlapping names)
• Request class integration
• Backwards compatibility with copy_* methods
• URL object as input parameter

All existing tests pass with the new behavior.

Related Issues

Fixes #652 - params overrides query string in url

Breaking Changes

This is technically a breaking change in behavior, but it fixes unintuitive behavior that was likely causing bugs in user code. The new behavior is:

1. More intuitive and matches user expectations
2. Consistent with the popular requests library
3. Better handles real-world use cases

Users who were relying on the old replacement behavior can achieve the same result by manually constructing URLs without existing query parameters.