Fixes failing persistence spec with DocumentNotSaved
Fixes failing persistence spec with DocumentNotSaved

It would appear that the rethinkdb gem now raises
RethinkDB::RqlRuntimeError when one attempts to act on a document
that has been deleted.

Explicitly catching this error and re-raising
NoBrainer::Error::DocumentNotSaved returns us to expected behavior.

Closes #23

databasify/rethinker