Fix compatibility issue

Author: ilijastuden

src/ActiveCollab/EmailReplyExtractor/Extractor/iOSExtractor.php

@@ -15,12 +15,12 @@ protected function processLines()
       // strip default signature
       if ($match_string) {
         list ($default_signature, $cut_line) = self::getLinesFromEnd(1);
-        $default_signature = implode(null, $default_signature);
+        $default_signature = implode('', $default_signature);
         if (preg_match('/' . $match_string . '/is', $default_signature)) {
           $this->body= array_splice($this->body, 0, $cut_line);
         }
       }
 
       $this->stripSignature();
     }
-  }
+  }