Support multiple form data values to be associated with the same name

In HTTP requests involving forms, it is common to send multiple values associated with the same name name (using [FormData.append](https://developer.mozilla.org/en-US/docs/Web/API/FormData/append)). Currently, `Reflex.Dom.Xhr.FormData` does not support this.

I can make a pull request with a working patch that I am currently using.