backends: make NewObject return fs.ErrorIsDir if possible

This changes the interface to NewObject so that if NewObject is called
on a directory then it should return fs.ErrorIsDir if possible without
doing any extra work, otherwise fs.ErrorObjectNotFound.

Tested on integration test server with:

go run integration-test.go -tests backend -run TestIntegration/FsMkdir/FsPutFiles/FsNewObjectDir -branch fix-stat -maxtries 1

backend/dropbox/dropbox.go

@@ -624,6 +624,9 @@ func (f *Fs) getFileMetadata(ctx context.Context, filePath string) (fileInfo *fi
 	}
 	fileInfo, ok := entry.(*files.FileMetadata)
 	if !ok {
+		if _, ok = entry.(*files.FolderMetadata); ok {
+			return nil, fs.ErrorIsDir
+		}
 		return nil, fs.ErrorNotAFile
 	}
 	return fileInfo, nil