Make XHR FormData work with this encoding (was: JSONFormData object)

[roman01la]

There's always a thing about fetching form data to construct JSON object. We need an analogue to FormData object.

[darobin]

I don't believe that we need a specific new JSONFormData, but we do need to make sure that XHR does not override the enctype to be multipart/form-data when FormData is built from a form.

[Nijso]

I no longer submit, I claim all that was and that is,