[1.3.X] Fixed `LayerMapping` to support `BigIntegerField`.
[1.3.X] Fixed `LayerMapping` to support `BigIntegerField`.

Backport of r15961 from trunk.

git-svn-id: 6bee183d120e8e569924b918ee6c564bbb314a32@15962 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django-nonrel/django-nonrel,Instagram/django,django-nonrel/django-nonrel,Instagram/django,django-nonrel/django-nonrel,Instagram/django