fix native-formdata has detection on ios 6, fixes #16022

This PR addresses the has("native-formdata") test that is wrong on iOS 6. Indeed, on this platform, ```typeof FormData``` is ```object```, not function. (see https://bugs.dojotoolkit.org/ticket/16022.)

There's no testcase attached as the [tests/request/xhr.html] (https://github.com/dojo/dojo/blob/master/tests/request/xhr.html) testcase already contains a [test](https://github.com/dojo/dojo/blob/master/tests/request/xhr.html#L285) that is executed only if has("native-formdata") is true. When executed on iOS6, this test is not run, showing the faulty detection test. With the proposed fix, the test is executed (successfully).