HashWithIndifferentAccess#select working as intended

Before this commit, #reject returned a HashWithIndifferentAccess,
whereas #select returned a Hash. Now #select also returns a
HashWithIndifferentAccess.
Remove warnings on Ruby 2.1

Conflicts:
	activesupport/lib/active_support/hash_with_indifferent_access.rb
Fix OrderedHash.select to return self instance.