[Fix] Update implementation to not return undefined descriptors.

Per tc39/ecma262#593 (comment)

implementation.js

@@ -35,7 +35,10 @@ module.exports = function getOwnPropertyDescriptors(value) {
 
 	var O = ES.ToObject(value);
 	return reduce(getAll(O), function (acc, key) {
-		safePut(acc, key, getDescriptor(O, key));
+		var descriptor = getDescriptor(O, key);
+		if (typeof descriptor !== 'undefined') {
+			safePut(acc, key, descriptor);
+		}
 		return acc;
 	}, {});
 };

test/tests.js

@@ -103,4 +103,21 @@ module.exports = function (getDescriptors, t) {
 		});
 		st.end();
 	});
+
+	var supportsProxy = typeof Proxy === 'function';
+	t.test('Proxies that return an undefined descriptor', { skip: !supportsProxy }, function (st) {
+		var obj = { foo: true };
+		var fooDescriptor = Object.getOwnPropertyDescriptor(obj, 'foo');
+
+		var proxy = new Proxy(obj, {
+			ownKeys: function () {
+				return ['foo', 'bar'];
+			},
+			getOwnPropertyDescriptor: function (target, key) {
+				return Object.getOwnPropertyDescriptor(target, key);
+			}
+		});
+		st.deepEqual(getDescriptors(proxy), { foo: fooDescriptor }, 'object has no descriptors');
+		st.end();
+	});
 };