Fix defineProperty to work with Java7 Rhino JS (#207)

Apparently `Object.defineProperty(fn, 'prototype', { writable: false });` sets `fn.prototype` to `null` in Java7 Rhino JS. Naive version works, so update detection code to check that.

Author: xkr47

src/util.js

@@ -1,12 +1,13 @@
 const realDefineProp = (function () {
-        let sentinel = {};
+        let sentinel = function(){};
         try {
             Object.defineProperty(sentinel, 'a', {
                 get: function () {
                     return 1;
                 }
             });
-            return sentinel.a === 1;
+            Object.defineProperty(sentinel, 'prototype', { writable: false });
+            return sentinel.a === 1 && sentinel.prototype instanceof Object;
         } catch (e) {
             return false;
         }