The private ResponseWriter implementation in the stdlib implements this
interface to enable some optimizations around sendfile. The interface is
asserted for in io.Copy, probably the most common way to serve files, so
without this method we are missing out on those optimizations. As we're
only interested in writing the headers and capturing the number of bytes
written, we can just call io.Copy on the wrapped http.ResponseWriter and
it will do the right thing, calling ReadFrom if it's implemented and
doing a regular copy if not.
