Fix assertion

Author: tvdijen

src/SAML2/AuthnRequest.php

@@ -639,7 +639,7 @@ public function getAssertionConsumerServiceIndex()
      */
     public function setAssertionConsumerServiceIndex($assertionConsumerServiceIndex)
     {
-        Assert::nullOrString($assertionConsumerServiceIndex);
+        Assert::nullOrInteger($assertionConsumerServiceIndex);
 
         $this->assertionConsumerServiceIndex = $assertionConsumerServiceIndex;
     }