mail: Support 'multipart/relative' content type

Which seems to be used by recent versions of Andriod OS. It appears to be a
synonym of `multipart/related`

include/pear/Mail/mimeDecode.php

@@ -307,6 +307,7 @@ class Mail_mimeDecode extends PEAR
                 case 'multipart/digest':
                 case 'multipart/alternative':
                 case 'multipart/related':
+                case 'multipart/relative':
                 case 'multipart/mixed':
                     if(!isset($content_type['other']['boundary'])){
                         $this->_error = 'No boundary found for ' . $content_type['value'] . ' part';