Fix evaluation in case of first "undefind" argument (#732)

It turns out that [undefined, 1].join(',') results in ",1" instead
of "undefined,1". This causes a syntax error when trying to pass undefined
as a first argument to `page.evaluate` method.

Fixes #572.

lib/helper.js

@@ -28,7 +28,17 @@ class Helper {
       console.assert(args.length === 0, 'Cannot evaluate a string with arguments');
       return fun;
     }
-    return `(${fun})(${args.map(x => JSON.stringify(x)).join(',')})`;
+    return `(${fun})(${args.map(serializeArgument).join(',')})`;
+
+    /**
+     * @param {*} arg
+     * @return {string}
+     */
+    function serializeArgument(arg) {
+      if (Object.is(arg, undefined))
+        return 'undefined';
+      return JSON.stringify(arg);
+    }
   }
 
   /**