FhirPath function memberOf does not work correctly with string as input

[marcovisserFurore]

Describe the bug
The FhirPath function memberOf does not work correctly with string as input. For example the following FP expression on the resource Patient does not work:

Patient.address.country.memberOf('http://hl7.org/fhir/ValueSet/iso3166-1-2')

The type of country is a string and not a code or CodeableConcept.

Expected behavior
The FP function memberOf should also work with instances of type string.

Tip for the developer
In the inline switch of ElementNavFhirExtensions.MemberOf() the type string should also be possible case.