Support deep-merging HashWithIndifferentAccess.

[#2732 state:committed]

Signed-off-by: Jeremy Kemper <jeremy@bitsweat.net>

Conflicts:
	activesupport/lib/active_support/core_ext/hash/deep_merge.rb
Merge pull request #28 from square/backport-indifferent-deep-merge

Support deep-merging HashWithIndifferentAccess.