update filter creation to correctly handle nested dict filters

[jsh2134]

previously sending this filter

>>> filters = ~ Filter(ac_score=None, original_ac_score=None)
>>> print filters.filters
 [{'not': {'and': [('original_ac_score', None), ('ac_score', None)]}}]

would end up sending the following to the API:

["not","and"]

[davecap]

Nice catch