The new get_serializer_for_object wasn't reliable enough, since it would

only work in certain circumstances. Any time the JSON encoder was used

to serialize part of a resource, it'd end up using the wrong resource to

serialize.
Now, when WebAPIResource serializes a payload, it passes itself as a

special parameter for use during serialization. This is used as a handle

for determining what to call get_serializer_for_object on.
With this, it should now be possible to always override serialization

for any object, reliably.

RB and Djblets unit tests pass.
Used this in a resource successfully. It was failing before.