Add $internal_date to public function appendMessage

[lwcorp]

While PHP doesn't support OAuth natively, its native imap_append command has an important parameter called $internal_date:

If this parameter is set, it will set the INTERNALDATE on the appended message. The parameter should be a date string that conforms to the rfc2060 specifications for a date_time value.

Can you add a similar parameter to your public function appendMessage?