Fix fatal PHP error on line 55. "Unexpected ;"

For some reason a constant was missing from this line.

Changed
`$this->_dataObject->data->admin_domain = ;` 

to  

`$this->_dataObject->data->admin_domain = APP_MAIL_DOMAIN;`
Merge pull request #3 from Dayjo/master

Fix fatal PHP error on line 55. "Unexpected ;"