activesupport: Memoize name object allocation in CurrentAttributes.instance

Also, use a symbol key for a faster hash lookup.

activesupport/lib/active_support/current_attributes.rb

@@ -91,7 +91,7 @@ class CurrentAttributes
     class << self
       # Returns singleton instance for this class in this thread. If none exists, one is created.
       def instance
-        current_instances[name] ||= new
+        current_instances[current_instances_key] ||= new
       end
 
       # Declares one or more attributes that will be given both class and instance accessor methods.
@@ -150,6 +150,10 @@ def current_instances
           Thread.current[:current_attributes_instances] ||= {}
         end
 
+        def current_instances_key
+          @current_instances_key ||= name.to_sym
+        end
+
         def method_missing(name, *args, &block)
           # Caches the method definition as a singleton method of the receiver.
           #