Snapshot generator inadvertedly removing base's Slicing.Discriminator

[abelenthoven]

If a base profile contains a sliced element with a discriminator, deriving from this profile and not setting a discriminator there removes the discriminator in the snapshot.
In essence, this problem originates in the CopyTo method of ElementDefinition.SlicingComponent. The snapshot generator uses this method to merge the base and derived profile elements by cloning the base element and then using CopyTo to merge the derived element onto the base. CopyTo checks whether the derived Discriminator list is , which it never is since the Discriminator property (like all FHIR POCO list properties) is lazily initialized with a list. Because of that, the existing Discriminator list is replaced by an empty one from the derived element.