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.

Although reverse accessor clashes should be skipped query name can't be hidden.

Thanks to Ian Foote and Tim Graham for the review.

Added the Migration.atomic attribute which can be set to False
for non-atomic migrations.