CodeSystem FHIRPath expression is incorrect in validating unique concept code values

Once of the constraints of the `CodeSystem` resource is the following expression:

concept.code.combine($this.descendants().concept.code).isDistinct()

The use of the `$this` invocation is incorrect. The input collection that `$this.descendants()` will operate under will only return the extensions or `id` of `concept.code`. Please see discussion at: http://community.fhir.org/t/codesystem-constraint-fhir-path-expression-seems-to-always-be-false/1999

Recommend the following changes:

concept.code.combine(%resource.concept.descendants().select(concept).code).isDistinct()

or 

concept.code.combine(%resource.concept.descendants().concept.code).isDistinct()