Updated closing logic

lib/Connection.js

@@ -110,12 +110,13 @@ class Connection {
   }
 
   // TODO: write a better closing logic
-  close () {
+  close (callback = () => {}) {
     debug('Closing connection', this.id)
     if (this._openRequests > 0) {
-      setTimeout(() => this.close(), 1000)
+      setTimeout(() => this.close(callback), 1000)
     } else {
       this._agent.destroy()
+      callback()
     }
   }