Avoid reading id twice in AR::Core#hash

This can be a very hot method so we should avoid reading id twice. This
restores that optimization we were using before introducing composite
primary keys.

Author: jhawthorn

activerecord/lib/active_record/core.rb

@@ -611,7 +611,7 @@ def ==(comparison_object)
     def hash
       id = self.id
 
-      if primary_key_values_present?
+      if self.class.composite_primary_key? ? primary_key_values_present? : id
         self.class.hash ^ id.hash
       else
         super