collaborators custom role permission is not maintained

[haribhandari07]

Steps to reproduce

1. Share a file or folder to a user as custom role
2. Set collaborators custom permission on the shared file for the previously shared user one by one:

Eg: a) set Can share permission and save and then refresh the page

  b) set "Can change" permission and save and then refresh the page

  c) set ```Can create``` permission and save and then refresh the page

  d) Now, go to the collaborators permission of the shared file, it shows all permission has been unset

 e) set ```Can delete``` permission and save and then refresh the page

3. You can repeat this cycle as many times you like

Expected behaviour

Collaborators custom permissions must be maintanied once they have been set

Actual behaviour

Collaborators custom permissions are not maintained when they are set one by one

[PVince81]

possibly related with #1897