Use an Ember Resource styls `isFoo` method and test it.

We've added an `isEmpty` check to Ember Resource, and we want to test
that is't behavior is correct, and we want it to be consistent with
our other `isSomething(obj)` checks.
A better test description.
Merge pull request #39 from zendesk/josh/extract_is_empty_and_test_it

Use an Ember Resource styls `isFoo` method and test it.