use correct path in case of autowebp

if autowebp is enabled and detected the result path has to be different from the case when it wasn't detected.

I modified normalize_path in the same sense as https://github.com/thumbor/thumbor/blob/master/thumbor/result_storages/file_storage.py was changed with https://github.com/thumbor/thumbor/commit/ae9a150e8a2b771dd49b4137186e9fdfbea09733#diff-9c717a8aed3280d5be3ffc22fefbc5cf