innodb_redo_log_encrypt does not behave correctly with SET PERSIST

While testing PR-4556, below scenario was executed

1. Set current value of innodb-redo-log as master_key
2. Execute: set persist innodb_redo_log_encrypt=keyring_key;
   Observation:
   It actually persists the value as master_key and makes an entry in performance_Schema.persisted_variables
   I think this is happening because, we are executing the query successfully and issuing a warning. If instead the query had returned an error then it would not have persisted an incorrect value