Some of the new caching improvements neglected to pass keyword arguments
down to the ETag generation functions, which meant that any subclasses
of WebAPIResource that needed to generate URLs based off these would
fail.

We now pass kwargs in all places in this code path.

There was also a bug where kwargs was treated as an args list, meaning
we just passed the key names. This caused some confusing errors after
applying the real fixes.

Djblets, Review Board, and RBCommons unit tests pass.