Add #clear to mem_cache_store, flushes all caches

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@8916 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

activesupport/lib/active_support/cache/mem_cache_store.rb

@@ -55,6 +55,10 @@ module ActiveSupport
         raise "Not supported by Memcache"
       end
 
+      def clear
+        @data.flush_all
+      end
+
       private
         def expires_in(options)
           (options && options[:expires_in]) || 0