How to close a message box in code?

The `MessageBoxManager.Get...` methods return and `IMsBox` that does not have a Close method. I have found issue #159 which says that this should work in new version, yet I cannot find a way to close it in the current 3.2.0.

I found that `MsBoxStandardView` has a Close method, but that is too not exposed in `IMsBox`.