Fixed SFTP storage to overwrite on rename

This fixes an issue that it's not possible to overwrite files because the part file can't be renamed to an existing file on a SFTP storage.

I don't like to need to call `is_dir()` and `file_exists()` but this is a quick fix just to make SFTP work correctly with WebDAV. We can improve the overall performance in OC7.

Please review @icewind1991 @DeepDiver1975 @karlitschek @schiesbn