Add a setter for NodeList.length

Apparently, Firefox's `Array.prototype.splice` sets `length`. This fails without
a setter, so add one that just returns the current value of `this._length`.

Author: lawnsea

lib/jsdom/level1/core.js

@@ -233,6 +233,9 @@ core.NodeList.prototype = {
     this._update();
     return this._length || 0;
   },
+  set length(length) {
+    return this._length;
+  },
   item: function(index) {
     this._update();
     return this[index] || null;