One option with Required doesn't need to be filled if another item of it...

Hi,
I was trying to configure two items in a mutual exclusive set and I would like to have one of them required. Marking both of them required would result in a deadlock where I couldn't provide both of them and at the same time were required to do so.

With this fix:
One option with Required doesn't need to be filled if another item of its MutualSet is provided. 
In another words: only one item of a MutualSet with all items Required need to be provided.

Please, check the unit tests for more detail.