Kubernetes `surge_upgrade` field cannot be set to `false`

tl;dr: The `SurgeUpgrade` field in [`KubernetesClusterUpdateRequest`](https://github.com/digitalocean/godo/blob/b16feac9513a6a1af15d33e1134631789e6fe20f/kubernetes.go#L90) should not have `omitempty` in the struct tags. This is preventing users from turning off `surge_upgrade` after the K8S cluster has been created.

Reproduction steps:
1. Create K8S cluster in DO. Make sure the `surge_upgrade` field is set to true
2. Using godo, update the cluster to disable `surge_upgrade`

Expected result:
`surge_upgrade` should be turned off

Actual result:
`surge_upgrade` is still on

It may be worthwhile looking for other non-pointer booleans with `omitempty` on them as well to check if they have the same issue.