Always be callin 'resp.Body.Close()

The response bodies needs to be closed regardless of whether we read it or
not. Always defer this immediately after calling RoundTripCheckError().

Except for waitForBackend() where we loop several times and don't want to
wait until the end to close all of the bodies.
Merge pull request #81 from alphagov/close_resp_body

Always be callin 'resp.Body.Close()