Fixes #39

Author: Bill Barnes

package.json

@@ -1,6 +1,6 @@
 {
   "name": "botframework-directlinejs",
-  "version": "0.9.13",
+  "version": "0.9.14",
   "description": "client library for the Microsoft Bot Framework Direct Line 3.0 protocol",
   "main": "built/directLine.js",
   "types": "built/directLine.d.ts",

src/directLine.ts

@@ -615,7 +615,7 @@ export class DirectLine implements IBotConnection {
                 // If we periodically ping the server with empty messages, it helps Chrome
                 // realize when connection breaks, and close the socket. We then throw an
                 // error, and that give us the opportunity to attempt to reconnect.
-                sub = Observable.interval(timeout).subscribe(_ => ws.send(null));
+                sub = Observable.interval(timeout).subscribe(_ => ws.send(""));
             }
 
             ws.onclose = close => {