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

Revert "CHEF-4101: DeepMerge - support overwriting hash values with nil"
Conflicts:
	spec/unit/mixin/deep_merge_spec.rb
Merge pull request #2388 from opscode/sersut/port-nil-attr-revert

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