def test_multiline_header_folding(standard_app: DevServerClient) -> None:
    """A header value can be split over multiple lines with a leading
    tab. The server will remove the newlines and preserve the tabs.

    https://tools.ietf.org/html/rfc2616#section-2.2
    """
    # conn.request doesn't support multiline values.
    conn = standard_app.connect()
    conn.putrequest("GET", "/")
    conn.putheader("XYZ", "first", "second", "third")
    conn.endheaders()
    r = conn.getresponse()
    data = json.load(r)
    r.close()
    assert data["HTTP_XYZ"] == "first\tsecond\tthird"
A header value can be split over multiple lines with a leading
    tab. The server will remove the newlines and preserve the tabs.

    https://tools.ietf.org/html/rfc2616#section-2.2
    
tests/test_serving.py