resumablerequestreader: allow initial response

Make it possible to inspect an initial response and pass it to
ResumableRequestReader. This makes it possible to inspect an initial
response and passing it to ResumableRequestReader to avoid making an
extra request.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
get layer: remove HEAD req & pass down response

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #6727 from unclejack/improve_bytes_range

Improve layer downloading