UserTore defined IdentityUser hard coded string but it sould be tkey

[jvelezc]

Currently:

    public class UserStore<TUser> : UserStore<TUser, IdentityRole, DbContext> where TUser : IdentityUser<string>, new()
    {
        public UserStore(DbContext context, IdentityErrorDescriber describer = null);
    }

Proposal:

public class UserStore<TUser> : UserStore<TUser, IdentityRole, DbContext> where TUser : IdentityUser<Tkey>, new()
    {
        public UserStore(DbContext context, IdentityErrorDescriber describer = null);
    }

Why? Because if I define

 public class ApplicationUser : IdentityUser<int>

I will subsequently not be able to use the userstore.

[rustd]

Duplicate of #585 which is tracking this.