UserManager.ThrowIfDisposed should be protected

[mwpowellhtx]

Hello,

In UserManager, ThrowIfDisposed should be protected. Today it is private.

protected void ThrowIfDisposed()
{
    if (_disposed)
    {
        throw new ObjectDisposedException(GetType().Name);
    }
}

Tangent to that is to expose an IsDisposed property.

Thank you.

[blowdart]

Linked to #750

[mwpowellhtx]

Thanks...

[HaoK]

41fa99e