[Cache/CouchbaseCache] Return false instead of null for compat.

This changeset fixes and verifies that instead of null, false is returned
from the fetch method. This fixes a bug which causes CouchbaseCache not
to work in combination with the ORM library. Test added.
[Cache] Refactoring testFalseOnFailedFetch into CacheTest

This changeset makes sure that all supported caches return
false and not null when a key is not found. This ensures
compatibility with the ORM layer.
Merge pull request #240 from daschl/couchbase-cache-orm-bugfix

[Cache/CouchbaseCache] Return false instead of null for compat.