Fix CustomResource apiVersion for the default group

CustomResources generated broken URLs for Kinds in the default API
group, like Services, ConfigMaps and so on. That lead to strange 403
responses by the API server.

kube/src/api/crds.rs

@@ -87,7 +87,11 @@ impl CrBuilder {
         let version = self.version.expect("Crd must have a version");
         let group = self.group.expect("Crd must have a group");
         CustomResource {
-            api_version: format!("{}/{}", group, version),
+            api_version: if group == "" {
+                version.clone()
+            } else {
+                format!("{}/{}", group, version)
+            },
             kind: self.kind,
             version,
             group,
@@ -157,6 +161,18 @@ mod test {
         assert_eq!(req.method(), "PATCH");
     }
 
+    #[test]
+    fn raw_resource_in_default_group() {
+        let r: Resource = CustomResource::kind("Service")
+            .group("")
+            .version("v1")
+            .into_resource();
+
+        let pp = PostParams::default();
+        let req = r.create(&pp, vec![]).unwrap();
+        assert_eq!(req.uri(), "/api/v1/services?");
+    }
+
     #[tokio::test]
     #[ignore] // circle has no kubeconfig
     async fn convenient_custom_resource() {