s3: fix upload of single files into buckets without create permission
s3: fix upload of single files into buckets without create permission

Before this change, attempting to upload a single file into an s3
bucket which did not have create permission gave AccessDenied: Access
Denied error when it tried to create the bucket.

This was masked until e2bf91452a263ff8babb3e5e153a2d2c7b4b3719 was
fixed.

This fix marks the bucket as OK if a fetch on an object indicates it
is OK. This stops rclone thinking it has to create the bucket in the
first place.

Fixes #4297

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone