If a callback variable was supplied for a view that errors (e.g. no
polygons), then this led to a cacheable (200) response which contained a
generator function (iterencode) in its _closable_objects attribute. The
cache could not then pickle this attribute and so raised an exception.

This was okay in previous versions because of a side effect of the
smart_bytes workaround for Django bug #24240 that was in place; this
was removed when support for Django prior to 1.8 was dropped.

Fix the issue by using an identity map (as map has no close() method and
so is not added to _closable_objects).

Don't add callback to non-JSON responses, or to responses that already
have a callback (presumably returned from a cache).