Fixed behavior of *_delete for DummyBackend

Previous (untested) version always failed with 404 after
successfully removing the resource. The problem was introduced
by recent changes in *_get which started raising errors instead
of returning nils.
Fixed the loading of fixtures for DummyBackend

Introduced a difference between flushed and emptied fixtures.
Helps to provide isolated testing. Emptied fixtures will stay
empty until they expire and flushed fixtures will be automatically
reloaded from JSON files.
Added the first batch of controller test

(mostly the same, apply DRY)