COM_CHANGE_USER rejected by MaxScale

If the client doesn't send the CLIENT_CONNECT_WITH_DB flag in the initial connection but later on does a COM_CHANGE_USER with a default database, MaxScale will reject the COM_CHANGE_USER and close the session.

It seems that the MariaDB documentation for COM_CHANGE_USER mistakenly stated that the database field is optional when in fact it is always sent in the COM_CHANGE_USER. The solution is to always parse the database field.