Update Refreshable hook and helper methods to get/set values as strings again

[strouptl]

@strzibny, I have identified a regression in the new Refreshable functionality, where the "credentials_refreshed_at" session value is not available to the needs_credentials_refresh? method, as Rack converts symbols to strings when storing session values. This causes the user to redirect continually to the otp/credentials/refresh page when trying to view the otp/token page since warden.session[:credentials_refreshed_at] remains nil.

I tried converting the symbols to strings where needed within the helper mehtods (e.g. needs_credentials_refresh?), but then the tests were failing. The reason for this appears to be that the test environment still get/sets symbols as symbols in the Rack session (as opposed to converting them to strings).

A simple solution is to utilize strings again for all get/set methods as before. This PR makes this change, and resolves the redirect issue mentioned above.

Leads:

• Rack Session https://www.rubydoc.info/gems/rack/1.6.0/Rack%2FSession%2FAbstract%2FSessionHash:[]=
• Rack Test Session: https://www.rubydoc.info/github/brynary/rack-test/Rack/Test/Session#env-instance_method