Make AutomailerSpool extend Swift_FileSpool

So that swiftmailer:spool:send does recover operation for automailer as well.

This might be a bit ugly since automailer is not a file based spool. The proper way is for swiftmailer to provide a RecoverableSpoolInterface so that FileSpool and AutomailerSpool implements it, then tries to recover in the SendMailCommand if the spool implements that interface.

Nevertheless, this should allow swiftmailer:spool:send command to do recovery for AutomailerSpool without any side effects.