Support defaultdict for namespace mapping

[nathanalderson]

In my use case, I want to ignore all namespaces. I hoped I could do this by using a defaultdict which returned None for all keys, but two minor issues in the code (see comments inline) prevented this from working as you would expect.

With this change, the following works:

>>> import collections
>>> xml = """
... <root xmlns="http://defaultns.com/"
...       xmlns:a="http://a.com/"
...       xmlns:b="http://b.com/">
...   <x>1</x>
...   <a:y>2</a:y>
...   <b:z>3</b:z>
... </root>
... """
>>> namespaces = collections.defaultdict(lambda: None)
>>> xmltodict.parse(xml, process_namespaces=True, namespaces=namespaces) == {
...     'root': {
...         'x': '1',
...         'y': '2',
...         'z': '3',
...     },
... }
True

[nathanalderson]

Although a defaultdict essentially has infinite items, until an item is actually inserted (either explicitly or on first access), it has zero length and evaluates to False.

[nathanalderson]

From the documentation on defaultdict:

Note that __missing__() is not called for any operations besides __getitem__(). This means that get() will, like normal dictionaries, return None as a default rather than using default_factory.

While a bit more verbose, this change uses __getitem__() instead of get(), so the defaultdict behaves as expected.

[martinblech]

Great contribution, thanks a lot!