Fix 'A non well formed numeric value encountered' ErrorException (#327)

$partNumber is allowed to be a string, therefore '|' will lead to an
Exception.

Author: Hokan22

src/IMAP/Message.php

@@ -690,7 +690,11 @@ private function createBody($type, $structure, $partNumber){
     private function fetchPart($structure, $partNumber){
         $encoding = $this->getEncoding($structure);
 
-        $content = \imap_fetchbody($this->client->getConnection(), $this->uid, $partNumber | 1, $this->fetch_options | IMAP::FT_UID);
+        if (!$partNumber) {
+            $partNumber = 1;
+        }
+
+        $content = \imap_fetchbody($this->client->getConnection(), $this->uid, $partNumber, $this->fetch_options | IMAP::FT_UID);
         $content = $this->decodeString($content, $structure->encoding);
 
         // We don't need to do convertEncoding() if charset is ASCII (us-ascii):