fixes #1217: edge case with default watchOptions value (#1218)

There is a problem when watchOptions is set to undefined in the options object. this.watchOptions is then set to undefined. We want an empty object so we can check poll property later.

Author: birdwell

lib/Server.js

@@ -50,7 +50,7 @@ function Server(compiler, options) {
   this.allowedHosts = options.allowedHosts;
   this.sockets = [];
   this.contentBaseWatchers = [];
-  this.watchOptions = options.watchOptions;
+  this.watchOptions = options.watchOptions || {};
 
   // Listening for events
   const invalidPlugin = () => {