Writers should be able to simply return the output instead of pushing it to a stream

[kaystrobach]

one of the code positions is here:
https://github.com/PHPOffice/PHPExcel/blob/develop/Classes/PHPExcel/Writer/Excel2007.php#L405

e.g. if filename is php://return the function should do a

 $buffer = file_get_contents($pFilename);
 @unlink($pFilename);
 return $buffer;

This way the integration in frameworks, which capture the output to avoid problems is a lot easier, as we also avoid tempfile handling ...