Define nested_under_indifferent_access to return self

This is required to make TaintedHash play nicely with
ActiveSupport::HashWithIndifferentAccess on Rails 3.2, which calls
nested_under_indifferent_access rather than with_indifferent_access on
Hash subclasses.
Merge pull request #8 from github/rails-3-2

Define nested_under_indifferent_access to return self