XHR FormData Safari fix

XHR requests submitted with the `options.data` property set to an instance of a `FormData` object fails in Safari. Safari reports the type of `FormData` as `object` while other browsers return it as `function`. This leads to the fourth parameter passed to util.parseArgs at `dojo/request/xhr.js:133` as `false`. Thus, the FormData object had later been turned into a string at `dojo/request/util.js:125`.

This fix prevents the above scenario from happening by modifying the `has('native-formdata')` test.