Case-insensitive username while login

As a User I don't like bothering myself always entering exact username while login, I'd like to be able to enter username e.g. with lower case letters

Acceptance Criteria:

• When user logs in first time, Poulpe will provide user details in case-insensitive manner, but Antarcticle should put it into DB exactly how Poulpe returned username.
• If user enters the username in different case, we should authenticate him if password matches.
• If password doesn't match, we should ask Poulpe (as usually) to authenticate user using exactly the same username he specified.
• If Poulpe returns a user with the same username as we already have in DB, but with different case, that should be treated as a different user.
• Now because we have 2 users with the same username, next time user logs in we should check his username in case-insensitive manner.