Fix building API docs due to the Djblets object caching change.

Djblets's WebAPIResource now sets request._djblets_webapi_object_cache
when being called as a view. However, the webapi doc generation code
bypasses this, resulting in a breakage when building docs.

We now set this on our DummyRequest manually, in order to give the
Djblets functions something to work with.

Reviewed at https://reviews.reviewboard.org/r/5918/