Revert "CHEF-4101: DeepMerge - support overwriting hash values with nil"

This reverts commit 972bc2f61685979e5b76f5481e3b1af2bd010d0e.

Conflicts:
	spec/unit/mixin/deep_merge_spec.rb
Merge pull request #2387 from opscode/sersut/revert-attr-nil-override

Revert "CHEF-4101: DeepMerge - support overwriting hash values with nil"