Fix LS_BUCKET

GET foo.s3.amazonaws.com/ and GET s3.amazonaws.com/foo should result in
an LS_BUCKET request, but under the previous logic it would result in a
LIST_BUCKETS request. GET s3.amazonaws.com/ still results in a
LIST_BUCKETS request due to the 'if path == "/" and s_req.is_path_style'
conditional.