s3: fix url decoding of NextMarker - fixes #3799

Before this patch we were failing to URL decode the NextMarker when
url encoding was used for the listing.

The result of this was duplicated listings entries for directories
with >1000 entries where the NextMarker was a file containing a space.

backend/s3/s3.go

@@ -1398,6 +1398,12 @@ func (f *Fs) list(ctx context.Context, bucket, directory, prefix string, addBuck
 		} else {
 			marker = resp.NextMarker
 		}
+		if urlEncodeListings {
+			*marker, err = url.QueryUnescape(*marker)
+			if err != nil {
+				return errors.Wrapf(err, "failed to URL decode NextMarker %q", *marker)
+			}
+		}
 	}
 	return nil
 }