DevelMailLog doesn't respect the MailInterface

DevelMailLog extends PhpMail and this one implements MailInterface

Accord with the MailInterface, the function mail should return a boolean.

* @return bool
* TRUE if the mail was successfully accepted for delivery, otherwise FALSE.

But as far as I can see in the code, the DevelMailLog::mail method returns:

return file_put_contents($output_file, $output);

Which accord with PHP's documentation:

This function returns the number of bytes that were written to the file, or FALSE on failure.

So when the mail is sent instead to return true it returns a number which can be confused and is not following the interface.

I will provide a test and a patch in my next comment.