Make login email case insensitive

When I create an account using capital letters in the email field, the email is saved in lower case in the database. So far everything is fine, but if I try to log in and use capital letters in the email, then my account cannot be found.

Example: I create an account with Nils.Mittler@mni.thm.de This works flawlessly at first. But if I want to login with Nils.Mittler@mni.thm.de, it doesn't work. If I use nils.mittler@mni.thm.de to login, then it works.