Problem

fetch() promises to add Content-Type: application/json for a mapping body only when the caller hasn't supplied a content type. In src/pythonnative/net.py, _build_request() uses the case-sensitive header_dict.setdefault("Content-Type", ...). A lowercase header creates a second entry, and urllib.request.Request normalizes both names, overwriting the caller's value.

Confirmed on the linked revision: a POST with headers={"content-type": "application/merge-patch+json"} and body={"name": "Ada"} produces [("Content-type", "application/json")] in request.header_items(). This can change how an API interprets the payload.

Requested change

Check for an existing content-type header case-insensitively before adding the JSON default. Keep the caller's supplied value and the existing JSON body encoding.

Acceptance criteria

• Parameterized tests cover Content-Type, content-type, CONTENT-TYPE, and mixed casing, preserving application/merge-patch+json in every case.
• The resulting request contains exactly one content-type header.
• A mapping body without a supplied content type still gets application/json.
• The caller's headers mapping is unchanged, and the request body still decodes to the original JSON object.

Starting points and verification

Extend tests/test_net.py. Construct requests with _build_request() for these tests; no server, device, or network is required. This issue concerns request header defaults; URL query assembly is a separate task.

uv run pytest tests/test_net.py -q
./scripts/check.sh

Suggested commit message

Following CONTRIBUTING.md:

fix(net): preserve custom content type regardless of header casing

Before starting

Please follow the issue-claiming process: check for an assignee or linked pull request, comment to claim the issue, and wait for a maintainer to assign it before opening a pull request. Contributor setup is in CONTRIBUTING.md.