Bump xml-common

Author: tvdijen

composer.json

@@ -14,7 +14,7 @@
         "ext-dom": "*",
 
         "simplesamlphp/assert": "~2.0",
-        "simplesamlphp/xml-common": "~2.7",
+        "simplesamlphp/xml-common": "~2.8",
         "simplesamlphp/xml-ws-policy": "~1.3",
         "simplesamlphp/xml-wss-core": "~1.4"
     },