Fix wrong use of Issuer-object

Author: tvdijen

src/SAML2/SOAPClient.php

@@ -107,7 +107,7 @@ public function send(Message $msg, Configuration $srcMetadata, Configuration $ds
         $context = stream_context_create($ctxOpts);
 
         $options = [
-            'uri' => $issuer,
+            'uri' => $issuer->getValue(),
             'location' => $msg->getDestination(),
             'stream_context' => $context,
         ];