Em.Resource.ajax properly supports a String argument

`jQuery.ajax('foo')` is shorthand for `jQuery.ajax({ url: 'foo' })`.
Unfortunately, `Em.Resource.ajax('foo')` didn't work because it tried
to manipulate the argument directly. This coerces the latter form to
the former so Ember-Resource can set the correct options.
Simplified `willFetch` stub
Merge pull request #74 from zendesk/shajith/fetch-spec-tidy

Tidy up the fetch spec a little
Merge branch 'master' into shajith/collection-fetch