PUT /fieldwork/:id now returns 204.

This is Rails 3.2's default responder behavior.  I personally don't care
that much about the exact status code of PUT /api/v1/fieldwork/:id and I
don't think Field cares either, so accepting the status change was fine
by me.

Note: the important part is that we get back a resolvable location to
check the merge status, and that, according to scenarios, still works.
ActiveRecord::StaleObjectError's constructor changed.
Speed up and simplify Survey specs.

This was originally motivated to clear out invalid references in test
data, but I think these specs are a bit easier to read, too.

Said test data still needs to be fixed or eliminated.
Rails 3.2.13.