CodeSystem/$validate-code does not understand the `url` parameter

[lawley]

This request with url= fails:
http://tx.fhir.org/r4/CodeSystem/$validate-code?url=http://snomed.info/sct&code=122298005

But this one, using the old STU3 parameter system succeeds:
http://tx.fhir.org/r4/CodeSystem/$validate-code?system=http://snomed.info/sct&code=122298005