Allow a Form::FileUpload to be passed in as a query value to #post

If an IO-like object is passed in as a query value to `Mechanize#post`, then Mechanize [will build a Form::FileUpload object for you](https://github.com/sparklemotion/mechanize/blob/master/lib/mechanize.rb#L493), and add it to the `form.file_uploads`. This is very convenient, except when you want to specify the `#mime_type` on said Form::FileUpload.

This patch allows the user to build their own `Form::FileUpload`, and pass that in as a query value.

Test included.