isUser() expert rule check is case sensitive

How to reproduce

• Create a user with a username that contains lower and uppercase characters, e.g. "JoeDoe"
• Make sure it is really persisted like this in the database (depends on database)
• Create course
• In the editor create an an expert access condition like this:

  isUser("JoeDoe")
  

  and

  isUser("joedoe")
  

• Login as JoeDoe, test rules
  => lowercase isUser check does not match