s3: Use rest.URLEscape rather than url.QueryEscape.
s3: Use rest.URLEscape rather than url.QueryEscape.

The X-Amz-Copy-Source takes a path. url.QueryEscape
escapes spaces with a plus sign while rest.URLEscape
(which mimics the url.PathEscape available from go 1.8)
uses '%20'

This works around an issue when copying objects with
spaces in their key on DigitalOcean Spaces.

olihey/rclone,olihey/rclone,seuffert/rclone,seuffert/rclone,ncw/rclone,olihey/rclone,olihey/rclone,ncw/rclone,ncw/rclone,B4dM4n/rclone,yonjah/rclone,B4dM4n/rclone,seuffert/rclone,seuffert/rclone,B4dM4n/rclone,ncw/rclone,yonjah/rclone,yonjah/rclone,yonjah/rclone,B4dM4n/rclone