Don't append newline after X-PHP-Originating-Script

This fixes [bug 66535](https://bugs.php.net/bug.php?id=66535).

When the extra headers are empty, no newline should be appended after X-PHP-Originating-Script.

This is also discussed in [this question](http://stackoverflow.com/questions/20832339/php-mail-adds-leading-newline-when-no-additional-headers-present).