Fix typo. php 5 errors out on this.

class/deliver/Deliver.class.php

@@ -452,7 +452,7 @@ class Deliver {
         }
         $header[] = $contenttype . $rn;
         if ($encoding = $rfc822_header->encoding) {
-            $header[] .= 'Content-Transfer-Encoding: ' . $encoding .  $rn;
+            $header[] = 'Content-Transfer-Encoding: ' . $encoding .  $rn;
         }
         if ($rfc822_header->dnt) {
             $dnt = $rfc822_header->getAddr_s('dnt');