Fix header injection in NativeMailerHandler

Hopefully attacker controlled data is never used to set the encoding
or content type, but just in case, prevent:

$nmh = new NativeMailerHandler($to, $subject, $from);
$nmh->setEncoding( "utf-8\r\nFrom: faked@attacker.org");

Since the injection happened in send(), there doesn't seem to be a good
way to add a unit test for this.