debug: log msg on socket connection

src/core/UppySocket.js

@@ -8,6 +8,7 @@ module.exports = class UppySocket {
     this.emitter = ee()
 
     this.socket.onopen = (e) => {
+      console.log('socket connection successfully opened.')
       this.isOpen = true
 
       while (this.queued.length > 0 && this.isOpen) {