'Split the value on \';\' incase the header includes extra attributes. E.g.
IE 6 is known to send:
If-Modified-Since: Sun, 25 Jun 2006 20:36:35 GMT; length=1506'

def __call__(self, *args, **kwargs):

    return _DateHeader.__call__(self, *args, **kwargs).split(';', 1)[0]
