MemCacheStore - Instance not Local in Decrement

active_support/cache/mem_cache_store.rb does operations on a instance variable representing the cache, but the decrement method is trying the operation on an undefined local.

also, noticed that activesupport/test/caching_test.rb doesn't have implementation specific tests specified, and doesn't seem to be fully fleshed out ?

will gladly write a test for this, but wanted to confirm the above before i start to restructure things in the tests :)