Fix URL for sendBeacon. (#980)

* Fix URL for sendBeacon.

* Performs sendBeacon once or connection.disconnect().

modules/xmpp/xmpp.js

 
                     // This is needed in some browsers where sync xhr sending
                     // is disabled by default on unload
-                    if (navigator.sendBeacon) {
+                    if (navigator.sendBeacon && !this.connection.disconnecting
+                            && this.connection.connected) {
+                        this.connection._changeConnectStatus(Strophe.Status.DISCONNECTING);
+                        this.connection.disconnecting = true;
+
                         const body = this.connection._proto._buildBody()
                             .attrs({
                                 type: 'terminate'
                         body.cnode(pres.tree());
 
                         const res = navigator.sendBeacon(
-                            `https${this.connection.service}`,
+                            `https:${this.connection.service}`,
                             Strophe.serialize(body.tree()));
 
                         logger.info(`Successfully send unavailable beacon ${res}`);
+
+                        this.connection._proto._abortAllRequests();
+                        this.connection._doDisconnect();
+
+                        return;
                     }
                 }
             }