Date attributes were set to null with findAll

With the current version of the adapter, `DS.attr('date')` attributes are set to null when the records they are part of were found using `findAll`. Turns out that this was because the serializer's `extractArray` method was normalizing each record twice.

This PR includes tests for loading date attributes in both the single record and array cases. (Single records were already working, but didn't have a test here.) There's also a small change that was necessary to get the adapter's integration tests to run at all. However, as noted in #14, there are a bunch of other failures. AFAICT, this PR does not make any of them worse.