Accept SOAP requests with no cwmp:ID element

Resolves #233

lib/soap.js

@@ -463,7 +463,9 @@ function request(httpRequest, cwmpVersion, warn) {
     }
   }
 
-  rpc.id = xml.get("/soap-env:Envelope/soap-env:Header/cwmp:ID", NAMESPACES[rpc.cwmpVersion]).text();
+  let idElement = xml.get("/soap-env:Envelope/soap-env:Header/cwmp:ID", NAMESPACES[rpc.cwmpVersion]);
+  if (idElement)
+    rpc.id = idElement.text();
 
   if (methodElement && !(XML_IGNORE_NAMESPACE && methodElement.name() === "Fault")) {
     switch (methodElement.name()) {