Merge pull request #2387 from opscode/sersut/revert-attr-nil-override

This reverts commit 972bc2f61685979e5b76f5481e3b1af2bd010d0e. This commit needs to be reverted in favor of newly merge attr changes that makes it possible to delete attr & attr subtrees.

Conflicts:
	spec/unit/mixin/deep_merge_spec.rb