[RaphaelFakhri]

When you pass a dict to data=, the Python client sets Content-Type: application/x-www-form-urlencoded on the request. It does this unconditionally, so a Content-Type header you pass in headers= gets replaced, for example application/x-www-form-urlencoded; charset=utf-8. If your header key uses a different case, such as content-type, both entries end up in the request header map, and which one is sent depends on the hash map iteration order.

This change adds the default Content-Type only when the request headers don't already contain one (compared case-insensitively). This matches httpx, where an explicit Content-Type takes precedence over the one derived from data=.

Changes

• impit-python/src/request.rs: in to_body, skip the default form Content-Type if the caller already set the header.
• impit-python/test/basic_client_test.py, impit-python/test/async_client_test.py: add test_form_keeps_custom_content_type. The tests send a form with a custom Content-Type to the local echoing server and check the header that arrives.

Testing

• Without the fix, both new tests fail: the echoed request contains content-type: application/x-www-form-urlencoded without the charset parameter.
• With the fix, both new tests pass.

Fixes #582

[barjin]

lgtm, thank you @RaphaelFakhri!