Rename userName parameter of IUserStore.SetNormalizedUserNameAsync method

[tugberkugurlu]

Here, userName parameter name should be normalizedUserName or something similar as this parameter is intended to be the normalized user name. userName is too confusing IMO.

Task SetNormalizedUserNameAsync(TUser user, string userName, CancellationToken cancellationToken = default(CancellationToken));

[HaoK]

Yep good catch, will fix