Also added tests for HStoreField and JSONField.
Thanks Aleksey Bukin for the report and Tim Graham for the initial patch and
the review.

Otherwise the migrations package won't be included in the tarball.

Thanks Simon Charette for the test.

Fix typo in comment

Parsed the CONTENT_TYPE header once and recorded it on the request.

Made sure the test doesn't try to aggregate over MultiPolygonField and made
AreaField turn decimals into floats on the way from the DB.

Thanks Daniel Wiesmann, Jani Tiainen, and Tim Graham for review and discussion.