Skaffold deploy not passing flags to Helm

I have tried the following variations to send the `--history-max 1` flag to Helm:

```yaml
deploy:
helm:
releases:
- name: my-app
flags:
upgrade: ["--history-max", "1"]
```

```yaml
deploy:
helm:
releases:
- name: my-app
flags:
upgrade: ["--history-max=1"]
```

```yaml
deploy:
helm:
releases:
- name: my-app
flags:
upgrade:
- "--history-max=1"
```

However, Skaffold ignores this option:
`Running command: [helm --kube-context gke_gcp-my-cluster-dev upgrade my-app skaffold/helm --post-renderer /usr/bin/skaffold --namespace test -f skaffold/helm/values/globals.yml -f skaffold/helm/values/values-dev.yml]`

What is the correct way to pass flags to Helm upgrade?