ensure prototype method polyfills dont affect config checking

Author: guybedford

lib/core.js

@@ -307,7 +307,8 @@ SystemJSLoader.prototype.config = function(cfg) {
     var hasConfig = false;
     function checkHasConfig(obj) {
       for (var p in obj)
-        return true;
+        if (hasOwnProperty.call(obj, p))
+          return true;
     }
     if (checkHasConfig(loader.packages) || checkHasConfig(loader.meta) || checkHasConfig(loader.depCache) || checkHasConfig(loader.bundles) || checkHasConfig(loader.packageConfigPaths))
       throw new TypeError('Incorrect configuration order. The baseURL must be configured with the first SystemJS.config call.');