def test_multiple_headers_concatenated(standard_app: DevServerClient) -> None:
    """A header key can be sent multiple times. The server will join all
    the values with commas.

    https://tools.ietf.org/html/rfc3875#section-4.1.18
    """
    # conn.request doesn't support multiple values.
    conn = standard_app.connect()
    conn.putrequest("GET", "/")
    conn.putheader("XYZ", "a ")  # trailing space is preserved
    conn.putheader("X-Ignore-1", "ignore value")
    conn.putheader("XYZ", " b")  # leading space is collapsed
    conn.putheader("X-Ignore-2", "ignore value")
    conn.putheader("XYZ", "c ")
    conn.putheader("X-Ignore-3", "ignore value")
    conn.putheader("XYZ", "d")
    conn.endheaders()
    r = conn.getresponse()
    data = json.load(r)
    r.close()
    assert data["HTTP_XYZ"] == "a ,b,c ,d"
A header key can be sent multiple times. The server will join all
    the values with commas.

    https://tools.ietf.org/html/rfc3875#section-4.1.18
    
tests/test_serving.py