rails/rails — pull request #8467 by gsphanikumar (2012-12-09 07:43:08+00:00)
Title: Fixed a method redefine warning 

While running the tests in activemodel the below warning is fixed.

/rails/activesupport/lib/active_support/core_ext/hash/deep_merge.rb:16: warning: method redefined; discarding old deep_merge!

I am not sure if using undef in the source is fine.