Make sure that OpenAPI still works when there are unimplemented
CRD keywords present.

#version: openapi

-- schema.yaml --
components:
  schemas:
    User:
      description: "A User uses something."
      type: array
      items:
        type: string
      x-kubernetes-list-type: set

-- out/decode/extract --
// A User uses something.
#User: [...string]
