Check to see if the SignedRequest headers are set before accessing

PHP Notices were issued if the request doesn't contain the expected headers.
Now, null values are passed to loadFromArray(), where the values will fail validation.

Author: chrisminett

src/MessageBird/Objects/SignedRequest.php

@@ -51,8 +51,10 @@ public static function createFromGlobals()
     {
         $body = file_get_contents('php://input');
         $queryParameters = $_GET;
-        $requestTimestamp = $_SERVER['HTTP_MESSAGEBIRD_REQUEST_TIMESTAMP'];
-        $signature = $_SERVER['HTTP_MESSAGEBIRD_SIGNATURE'];
+        $requestTimestamp = isset($_SERVER['HTTP_MESSAGEBIRD_REQUEST_TIMESTAMP']) ?
+            $_SERVER['HTTP_MESSAGEBIRD_REQUEST_TIMESTAMP'] : null;
+        $signature = isset($_SERVER['HTTP_MESSAGEBIRD_SIGNATURE']) ?
+            $_SERVER['HTTP_MESSAGEBIRD_SIGNATURE'] : null;
 
         $signedRequest = new SignedRequest();
         $signedRequest->loadFromArray(compact('body', 'queryParameters', 'requestTimestamp', 'signature'));