DRY up SnapshotsController#take_snapshot specs
DRY up SnapshotsController#take_snapshot specs

In each of the examples, we were stubbing
SnapshotterWorker.perform_async and performing a POST request. I DRYed
this up by moving the stubbing into a before block and the request and
response into a subject.

Change-Id: I46b33b15290bba06b587b79adcc44af27e989df3

diffux/diffux,diffux/diffux,diffux/diffux,kalw/diffux,kalw/diffux,kalw/diffux