Get the proper UID

Some user backends (like the database backend) allow us to obtain a user
case insensitive. However the UID itself is case sensitive.

Example:
* create a user User1
* login as User1
  - This results the data/User1 folder to be created etc
* now have some code somewhere that obtains the userFolder (from
IRootFolder) but pas in 'uSER1' as uid
  - The code will check if that is a valid user. And in this case it is
  since User1 and uSER1 both map to the same user
  - However the the UID in the user object is used for the folder a new
  folder fill be create data/uSER1

With this PR this is avoided now. Since we obtain the real UID casing in
the backend before creating the user object.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>