fix: allow content_type with a list of header tuples in add()

[breken-ai]

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update
• Other

Description

A list of (name, value) tuples is the only way to give a mocked response repeated headers, such as two Set-Cookie headers. The repo already tests that form in test_response_cookies_session. If you pass content_type along with it, responses.add() crashes before registering anything:

responses.add(
    responses.GET,
    "http://example.com/",
    body="<p>ok</p>",
    content_type="text/html",
    headers=[("Set-Cookie", "a=1"), ("Set-Cookie", "b=2")],
)
# AttributeError: 'tuple' object has no attribute 'lower'

The Content-Type conflict check in add() calls .lower() on each item of headers, and that only works when the items are names from a dict. This PR does the check with HTTPHeaderDict, the same way get_headers() already does. HTTPHeaderDict accepts a mapping or a list of tuples and compares names case-insensitively. A list that really does contain a content-type tuple still raises the existing RuntimeError.

Related Issues

• None. I found no existing issue or PR for this.

PR checklist

Before submitting this pull request, I have done the following:

• Read the contributing guidelines
• Ran tox and pre-commit checks locally. I ran pre-commit on the changed files and pytest on Python 3.12 (urllib3 2) and Python 3.9 (urllib3 1.26): 253 passed. mypy shows the same 3 errors as master and none new.
• Added my changes to the CHANGES file

Added/updated tests?

Current repository has 100% test coverage.

• Yes. test_content_type_with_list_of_header_tuples fails on master with the AttributeError above and passes with this change. It also checks that a conflicting content-type tuple still raises RuntimeError.
• No, and this is why:
• I need help with writing tests

Disclosure: an AI coding agent (Claude) on the breken-ai account found this bug, wrote the fix and test, and ran the checks listed above.