Summary

_build_request() used header_dict.setdefault("Content-Type", "application/json") when encoding a mapping body. That lookup is case-sensitive, so a caller-supplied content-type / CONTENT-TYPE header was ignored and urllib.request.Request later collapsed both names, overwriting the custom value with application/json.

This change only adds the JSON default when no existing header name matches content-type case-insensitively. The caller mapping is copied first, so it is left unchanged.

Fixes #87

Motivation

Issue #87 documents that headers={"content-type": "application/merge-patch+json"} with a dict body produced Content-type: application/json on the request. That can change how an API interprets the payload.

Implementation

In src/pythonnative/net.py, replace setdefault("Content-Type", ...) with:

if not any(name.lower() == "content-type" for name in header_dict):
    header_dict["Content-Type"] = "application/json"

JSON encoding of mapping bodies is unchanged.

Testing

Added parameterized tests in tests/test_net.py that construct requests with _build_request() (no network):

• Content-Type, content-type, CONTENT-TYPE, and Content-type all keep application/merge-patch+json
• exactly one content-type header remains on the request
• the original headers mapping is unchanged
• the body still decodes to the original JSON object
• a mapping body with no content-type still gets application/json

Checks performed in this environment:

• Isolated reproduction of _build_request header assembly on Python 3.12 (project requires 3.13 for the full package import). Result: all four casings preserved a single application/merge-patch+json header; default path still sets application/json.
• Full uv run pytest tests/test_net.py -q and ./scripts/check.sh were not run here because Python 3.13 is not available on this runner. Please rely on CI for those.