handle CaseInsensitiveDict when dumping to json

fixes #3
handle CaseInsensitiveDict with convert_to_dict

The convert_to_dict function walks a tree of dictionaries and converts
everything with an "items" attribute into a regular dictionary. This is
able to pick up on embedded CaseInsensitiveDict instances.

Also, this includes tests of convert_to_dict to confirm that it's
working.

really fixes #3
Merge branch 'fix-case-insensitive-dict-serialization' into master

This fixes a bug with CaseInsensitiveDict in requests related to
serialization.

version bump to: v0.0.7

see #3