ensure return_to location is properly stored

Fixes small typo in session key for what Devise expects when retrieving stored `return_to` path for a given scope. 

[This scenario](https://github.com/Houdini/two_factor_authentication/blob/master/spec/features/two_factor_authenticatable_spec.rb#L33) actually already covers this issue, but the explicit call to `store_location_for` in the spec rails_app `HomeController` was masking the bug. Removing that filter gives proper coverage.