Test failure

[fabaff]

Checklist

• The bug is reproducible against the latest release and/or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

Test failure on NixOS and Fedora during the package build process.

============================= test session starts ==============================
platform linux -- Python 3.8.7, pytest-6.1.2, py-1.9.0, pluggy-0.13.1
rootdir: /build/source, configfile: setup.cfg
plugins: asyncio-0.14.0, cov-2.10.1
collected 240 items                                                            
[...]
=================================== FAILURES ===================================
_______________________________ test_debug_html ________________________________

    def test_debug_html():
        async def app(scope, receive, send):
            raise RuntimeError("Something went wrong")

        app = ServerErrorMiddleware(app, debug=True)
        client = TestClient(app, raise_server_exceptions=False)
        response = client.get("/", headers={"Accept": "text/html, */*"})
        assert response.status_code == 500
>       assert response.headers["content-type"].startswith("text/html")

tests/middleware/test_errors.py:42:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = {}, key = 'content-type'

    def __getitem__(self, key):
>       return self._store[key.lower()][1]
E       KeyError: 'content-type'

/nix/store/9bb3iygm8kkdzfsaj3j1j2q845nzja1p-python3.8-requests-2.25.0/lib/python3.8/site-packages/requests/structures.py:54: KeyError

Fedora uses different releases

============================= test session starts ==============================
platform linux -- Python 3.9.1, pytest-6.0.2, py-1.10.0, pluggy-0.13.1 -- /usr/bin/python3
cachedir: .pytest_cache
benchmark: 3.2.3 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
rootdir: /home/python-starlette/starlette-0.14.1, configfile: setup.cfg
plugins: cov-2.10.1, asyncio-0.14.0, requests-mock-1.7.0, httpx-0.10.0, mock-3.3.1, benchmark-3.2.3, aresponses-2.0.0, forked-1.3.0, xdist-1.34.0, django-3.8.0, vcr-1.0.2, datafiles-2.0, pyfakefs-3.5.8, httpbin-1.0.0, sugar-0.9.3, aiohttp-0.3.0
collecting ... collected 187 items
[...]

To reproduce

Run with enabled unit tests.

Expected behavior

All tests passing.

Actual behavior

Debugging material

Environment

• OS: NixOS and Fedora
• Python version: 3.8.7
• Starlette version: 0.14.1 but as also present in 0.13.8

Additional context