'Extend the headers with a dict or an iterable yielding keys and
values.'

def extend(self, iterable):

    if isinstance(iterable, dict):
        for (key, value) in iteritems(iterable):
            if isinstance(value, (tuple, list)):
                for v in value:
                    self.add(key, v)
            else:
                self.add(key, value)
    else:
        for (key, value) in iterable:
            self.add(key, value)
