fixed decoding of subject

imap_qprint seems to have problems with strings containing the character set (like described here: http://php.net/manual/de/function.imap-qprint.php#4009) - quoted_printable_decode works much better

Author: Alexander Reichardt

src/PhpImap/Mailbox.php

@@ -470,7 +470,7 @@ protected function initMailPart(IncomingMail $mail, $partStructure, $partNum, $m
 			$data = imap_base64($data);
 		}
 		elseif($partStructure->encoding == 4) {
-			$data = imap_qprint($data);
+			$data = quoted_printable_decode($data);
 		}
 
 		$params = array();