'update() extends rather than replaces existing key lists:
>>> a = MultiDict({\'x\': 1})
>>> b = MultiDict({\'x\': 2, \'y\': 3})
>>> a.update(b)
>>> a
MultiDict([(\'y\', 3), (\'x\', 1), (\'x\', 2)])
If the value list for a key in ``other_dict`` is empty, no new values
will be added to the dict and the key will not be created:
>>> x = {\'empty_list\': []}
>>> y = MultiDict()
>>> y.update(x)
>>> y
MultiDict([])'

def update(self, other_dict):

    for (key, value) in iter_multi_items(other_dict):
        MultiDict.add(self, key, value)
