Produce an appropriate Cookie header string to be sent with `request`, or None.

    :rtype: str

def get_cookie_header(jar, request):
    """
    Produce an appropriate Cookie header string to be sent with `request`, or None.

    :rtype: str
    """
    r = MockRequest(request)
    jar.add_cookie_header(r)
    return r.get_new_headers().get('Cookie')