Do not double decode values

The values are already decoded by the webserver and we do not need to double decode it.

Testplan:

- [x] Upload a file containing + and another one containing & in the filename, upload it to a folder and share the folder => Downloading both files alone from the sharing page should work
- [x] Upload a file containing + and another one containing & in the filename, upload it to a folder and share the folder => Downloading both files together from the sharing page should work (ZIP download!)
- [x] Do the same test with regular file names

Fixes https://github.com/owncloud/core/issues/11012