'converts ``values()`` into a string value
This method converts the results of ``values()`` into a string
value for common usage.  By default, it is asserted that only
one value exists; if you need to access all values then either
call ``values()`` directly, or inherit ``_MultiValueHeader``
which overrides this method to return a comma separated list of
values as described by section 4.2 of RFC 2616.'

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

    values = self.values(*args, **kwargs)
    assert isinstance(values, (tuple, list))
    if (not values):
        return ''
    assert (len(values) == 1), ('more    than    one    value:    %s' % repr(values))
    return str(values[0]).strip()
