Simplify update_testcase
    
    The sequential changes to this over time have left it kinda weird
    if you look at it with fresh eyes. If you don't pass it a URL it
    just does get_testcase, and if you do pass it a URL, it actually
    does exactly the same thing as create_testcase. So let's just
    make it explicitly work that way, and replace the unnecessary
    _KEEP thing with None.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>