Test io.Seeker items work with http.ServeContent
Test io.Seeker items work with http.ServeContent

net/http.ServeContent function takes an io.ReadSeeker, which can be
used to serve ranged content.

The function uses `Seek(0, io.SeekEnd)` to establish the total size of
the served content and then resets the seeker with `Seek(0, io.SeekStart)`.

Any stow item that returns an io.Seeker should be able to work with
those two methods.

graymeta/stow