POST sends a POST request using client to the given URL and unmarshal the
body into v if it's not nil. It will only send using multipart if files is
true.
type DataMultipartWriter interface {
// NeedsMultipart returns true if the data interface must be sent using// multipart form.
NeedsMultipart() boolhttputil.MultipartWriter
}
DataMultipartWriter is a MultipartWriter that also contains data that's
JSON-marshalable.