Make sure cached_class collection_name is string before calling singularize on it.
Make sure cached_class collection_name is string before calling singularize on it.

Mongoid 3.0 returns a symbol, which singularize chokes on.

Signed-off-by: Marc MacLeod <55017ab2bb95c30036d255be793d58bea987cbcb@gmail.com>

dblock/mongoid-cached-json