Fix JSONObjectField

When pulling data from the db, the JSONObjectField values weren't being
correctly converted back to Python types. Thus you couldn't save some
data, then later pull it out, add to it and put it back in.

This fixes that and tests that were testing against the wrong values.

This fixes some dumb stuff I did. It doesn't seem to affect existing db stuff,
but rather just fixes things coming out of the db.

r?
This was a goof. The default needs to be an empty Python dict--not the string representation of one.