Fix: use JID from credentials when credentials_url is set (#1638)

* Fix: async SetUserJID

src/headless/converse-core.js

@@ -671,11 +671,12 @@ function fetchLoginCredentials (wait=0) {
             xhr.onload = () => {
                 if (xhr.status >= 200 && xhr.status < 400) {
                     const data = JSON.parse(xhr.responseText);
-                    setUserJID(data.jid);
+                    setUserJID(data.jid).then(() => {
                         resolve({
                             jid: data.jid,
                             password: data.password
                         });
+                    });
                 } else {
                     reject(new Error(`${xhr.status}: ${xhr.responseText}`));
                 }