Fix ocs share api paths

Fix sharing results for reshared files.

It is not that obvious how to retrieve the correct Shared/-path for the current user in all circumstances (re-shares, re-re-shares, sharing of sub(sub)files, sharing a sub-folder and than re-share a sub-sub-file, etc)

I added some tests which hopefully covers most possibility. But some creative testing during review would be appreciated. :wink: 

fix #7662