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