Context manager doesn't work for ListDatabases

[controversial]

Using the context manager on a ListDatabase converts it to a dict:

>>> a = livejson.ListDatabase("a.json")
>>> a
[]
>>> with a:
...     a.append(0)
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/_abcoll.py", line 659, in append
    self.insert(len(self), value)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/livejson.py", line 265, in insert
    data.insert(index, value)
AttributeError: 'dict' object has no attribute 'insert'
>>> a
{}