Function write_formdata 

pub fn write_formdata<S: Write, W: Write>(
    stream: &mut S,
    boundary: &Vec<u8>,
    formdata: &mut FormData,
    w: Option<fn(name: &mut FilePart) -> Result<()>>,
) -> Result<usize, Error>

Stream out multipart/form-data body content matching the passed in formdata. This does not stream out headers, so the caller must stream those out before calling write_formdata().