fix(api-machinery): Fixes patching for unstructured objects

CRDs and other objects seen as unstructured cannot use strategic
merge patching. It has never been supported on CRDs. Previously,
cases like unstructured objects could have caused an unregistered
error. This is no longer the case.

This change explicitly looks for unstructured objects and handles
those using json merge patching.

Closes helm#3382

pkg/kube/client.go

@@ -399,14 +399,25 @@ func createPatch(mapping *meta.RESTMapping, target, current runtime.Object) ([]b
 		return nil, types.StrategicMergePatchType, fmt.Errorf("serializing target configuration: %s", err)
 	}
 
+	// While different objects need different merge types, the parent function
+	// that calls this does not try to create a patch when the data (first
+	// returned object) is nil. We can skip calculating the the merge type as
+	// the returned merge type is ignored.
 	if apiequality.Semantic.DeepEqual(oldData, newData) {
 		return nil, types.StrategicMergePatchType, nil
 	}
 
 	// Get a versioned object
 	versionedObject, err := mapping.ConvertToVersion(target, mapping.GroupVersionKind.GroupVersion())
+
+	// Unstructured objects, such as CRDs, may not have an not registered error
+	// returned from ConvertToVersion. Anything that's unstructured should
+	// use the jsonpatch.CreateMergePatch. Strategic Merge Patch is not supported
+	// on objects like CRDs.
+	_, isUnstructured := versionedObject.(runtime.Unstructured)
+
 	switch {
-	case runtime.IsNotRegisteredError(err):
+	case runtime.IsNotRegisteredError(err), isUnstructured:
 		// fall back to generic JSON merge patch
 		patch, err := jsonpatch.CreateMergePatch(oldData, newData)
 		return patch, types.MergePatchType, err