Pass in DbContextOptions to *.IdentityDbContext for all version of IdentityDbContext <>

[SychevIgor]

We need to configure dbcontext as it mentioned here: https://github.com/aspnet/EntityFramework/wiki/Configuring-a-DbContext

It's cool that we make it possible to pass DbContextOptions via constructor for this class:

  public class IdentityDbContext<TUser, TRole, TKey> : DbContext
        where TUser : IdentityUser<TKey>
        where TRole : IdentityRole<TKey>
        where TKey : IEquatable<TKey>

#570
#418

But for 2 others we still can't do it without modifications of our context:

    /// <summary>
    /// Base class for the Entity Framework database context used for identity.
    /// </summary>
    public class IdentityDbContext : IdentityDbContext<IdentityUser, IdentityRole, string> { }

    /// <summary>
    /// Base class for the Entity Framework database context used for identity.
    /// </summary>
    /// <typeparam name="TUser">The type of the user objects.</typeparam>
    public class IdentityDbContext<TUser> : IdentityDbContext<TUser, IdentityRole, string> where 
        TUser : IdentityUser { }

As a result you are forcing us to inherit our dbcontext from IdentityDbContext : IdentityDbContext<IdentityUser, IdentityRole, string> and making the other 2 others versions IdentityDbContext<TUser>, IdentityDbContext completely useless.

[SychevIgor]

Looks like somebody already propose this changes #643 and I hope that team will get it to a project.

[divega]

@HaoK @rustd I just looked at this bug and I agree we should just add the constructors to all the versions of IdentityDbContext to take the same parameters as the base. I am assigning it to @HaoK.

[HaoK]

ada5756