progress: support ReadCloser

[sumnerevans]

Signed-off-by: Sumner Evans [email protected]

[tulir]

Not sure if this needs to be separate? Reader could just support closing

func (r *Reader) Close() error {
  innerCloser, ok := r.inner.(io.ReadCloser)
  if ok {
    return innerCloser.Close()
  }
  return nil
}