Previously, a when uploading a new diff for a review request, if there

were webhooks configured, they would fail to execute because DiffSets

could not be serialized. Now that Djblets supports serialization of

arbitrary Django models, we implement this behaviour for DiffSets so

they can be serialized in web hook payloads (as their diffviewer URL).
Web hooks have been updated to use the ResourceAPIEncoder so if any

model present in a payload is registered with a resource, it will be

serialized accordingly. This should avoid future serialization issues

with web hooks.

Ran unit tests.
Could not reproduce issue 4042 with this patch applied.
The new unit test fails without this patch applied.