Use 1.9 way

Author: spastorino

Diff for: activerecord/lib/active_record/associations/collection_association.rb

@@ -400,12 +400,7 @@ def merge_target_lists(persisted, memory)
           return memory    if persisted.empty?
 
           persisted.map! do |record|
-            # Unfortunately we cannot simply do memory.delete(record) since on 1.8 this returns
-            # record rather than memory.at(memory.index(record)). The behavior is fixed in 1.9.
-            mem_index = memory.index(record)
-
-            if mem_index
-              mem_record = memory.delete_at(mem_index)
+            if mem_record = memory.delete(record)
 
               (record.attribute_names - mem_record.changes.keys).each do |name|
                 mem_record[name] = record[name]