Do not include unset parameters in query string

Fixes #1

Author: danmichaelo

src/Subscriber.php

@@ -148,16 +148,22 @@ private function change_subscription($mode, $topic_url, $http_function = false)
             throw new InvalidArgumentException('The specified topic url does not appear to be valid: ' . $topic_url);
         }
 
-        // set the mode subscribe/unsubscribe
+        // set the required parameters
         $post_string = 'hub.mode=' . $mode;
         $post_string .= '&hub.callback=' . urlencode($this->callback_url);
-        $post_string .= '&hub.verify=' . $this->verify;
-        $post_string .= '&hub.verify_token=' . $this->verify_token;
-        $post_string .= '&hub.lease_seconds=' . $this->lease_seconds;
-
-        // append the topic url parameters
         $post_string .= '&hub.topic=' . urlencode($topic_url);
 
+        // add any optional parameter
+        if (!is_null($this->verify)) {
+            $post_string .= '&hub.verify=' . $this->verify;
+        }
+        if (!is_null($this->verify_token)) {
+            $post_string .= '&hub.verify_token=' . $this->verify_token;
+        }
+        if (!is_null($this->lease_seconds)) {
+            $post_string .= '&hub.lease_seconds=' . $this->lease_seconds;
+        }
+
         // make the http post request and return true/false
         // easy to over-write to use your own http function
         if ($http_function) {