'Add all the headers from the iterable to the set.
:param iterable: updates the set with the items from the iterable.'

def update(self, iterable):

    inserted_any = False
    for header in iterable:
        key = header.lower()
        if (key not in self._set):
            self._headers.append(header)
            self._set.add(key)
            inserted_any = True
    if (inserted_any and (self.on_update is not None)):
        self.on_update(self)
