Before this change the rest package would forward all the headers on
an HTTP redirect, including the Authorization: header. This caused
problems when forwarded to a signed S3 URL ("Only one auth mechanism
allowed") as well as being a potential security risk.
After we use the go1.8+ mechanism for doing this instead of using our
own which does it correctly removing the Authorization: header when
redirecting to a different host.
This hasn't fixed the behaviour for rclone compiled with go1.7.
Fixes#2635
