IndifferentAccess, make #store respect indifference

Hash provides #store as essentially a synonym for #[]= -- but for some reason, in MRI at least, they have different implementations, neither is implemented in terms of the other. 

You forgot to fix :store for indifferent access. Fixed here with a simple alias method, which is the same [solution ActiveSupport::HashWithIndifferentAccess uses](https://github.com/rails/rails/blob/72f7f94cec42a841a17ab112af27579fcb4d9920/activesupport/lib/active_support/hash_with_indifferent_access.rb#L97).