List output with differing types should be more resilient

kubectl get can output a series of objects as a List in versioned
form, but not all API objects are available in the same schema.
Make the act of converting a []runtime.Object to api.List more
robust and add a test to verify its behavior in Get.

Makes it easier for client code to output unified objects.

hack/test-cmd.sh

@@ -162,6 +162,7 @@ for version in "${kube_api_versions[@]}"; do
   # Command
   kubectl create "${kube_flags[@]}" -f examples/limitrange/valid-pod.json
   # Post-condition: valid-pod POD is running
+  kubectl get "${kube_flags[@]}" pods -o json
   kube::test::get_object_assert pods "{{range.items}}{{$id_field}}:{{end}}" 'valid-pod:'
   kube::test::get_object_assert 'pod valid-pod' "{{$id_field}}" 'valid-pod'
   kube::test::get_object_assert 'pod/valid-pod' "{{$id_field}}" 'valid-pod'