Check for a valid encoding before using a public session id

[HectorMF]

When penetration testing I discovered that if a public session ID contains an invalid byte sequence the following error occurs:

Error during failsafe response: invalid byte sequence in UTF-8
/gems/activerecord-session_store-2.1.0/lib/action_dispatch/session/active_record_store.rb:163:in `private_session_id?'

This causes an internal server error, when it really should just handle it as an invalid id

[ashak]

I've just run into the same issue and ended up here.

Testing the suggested fix out locally appears to work and stops the errors.

Is there anything we can do to try to get this reviewed and merged for a new release?