fix the actual dict_diff for computing changes

archived_data when not to_json() was actually resolving datetimes as objects, not as pure json strings.

this addresses django 1.5+ microsecond resolution datetime comparisons which was passing equality checks before in the other configuration. now having apples-to-apples comparison, datetime precision trunctation  (due to json spec for timestamps) now works as expected.
Merge pull request #9 from dimagi/diff_fix

Diff fix