MemCacheStore does not work with Memcached::Rails

Lines 43-46 of activesupport/lib/active_support/cache/mem_cache_store.rb read:

  # Instead of addresses one can pass in a MemCache-like object. For example:
  #
  #   require 'memcached' # gem install memcached; uses C bindings to libmemcached
  #   ActiveSupport::Cache::MemCacheStore.new(Memcached::Rails.new("localhost:11211"))

However, actually trying to do this raises an ArgumentError ("wrong number of arguments (2 for 1)") since Memcached::Rails#delete (introduced in http://github.com/fauna/memcached/commit/0a5df4124076af585e726a372b...) only takes one argument.

Since memcached has deprecated support for an expiry on the delete operation in version 1.4, the memcache-client gem's MemCache#delete method ignores it in edge as well (as explained at http://github.com/mperham/memcache-client/blob/master/lib/memcache..... In previous versions, it provides a default argument value for expiry, so it supports calling the delete method with just the key.

Therefore, calling @data.delete(key) in MemCacheStore#delete is safe for usage with the memcache-client gem, and makes it usable with the memcached gem, as advertised (but not currently the case).

This patch adds a test that the underlying implementation is only called with one argument (the key) and changes the call.