The angular $httpBackend mocking service supports two primary means of
usage:

   .expect(...).respond(...)

and

   .when(...).respond(...)

The latter pattern allows a single handler to respond to multiple
requests, in any order. The latter is precise -- one .expect() must be
matched by one request, in the correct order.

This commit changes the tests for the "features" service to use this
more precise form of testing.