fix(symbols): check that descriptors is an object

In IE, createWithSymbols fails if descriptors is undefined, giving the following error:
"Object.getOwnPropertyNames: argument is not an Object"
This makes sure the item is an object before continuing.

src/symbol.js

@@ -49,11 +49,13 @@ if (typeof FEATURE_NO_ES2015 === 'undefined') {
     },
     createWithSymbols = function (proto, descriptors) {
       var self = create(proto);
-      gOPN(descriptors).forEach(function (key) {
-        if (propertyIsEnumerable.call(descriptors, key)) {
-          $defineProperty(self, key, descriptors[key]);
-        }
-      });
+      if (descriptors !== null && typeof descriptors === 'object') {
+        gOPN(descriptors).forEach(function (key) {
+          if (propertyIsEnumerable.call(descriptors, key)) {
+            $defineProperty(self, key, descriptors[key]);
+          }
+        });
+      }
       return self;
     },
     copyAsNonEnumerable = function (descriptor) {