API tests for deleting an item.
Created test DSL for dealing with pre-populated collections.
Reworked the test DSL for dealing with the response body.
Fixed bug in malformed? and processable? decisions for delete.
Implemented additional tests for create item fail cases.
Fixed a bug where "JSON" that was just a string wouldn't trigger the malformed decision.
Added validation of the Content-Type of the response to all API tests.