Actions API does not return `completed_at` param

[adamjezek98]

Steps to reproduce:

1. Start snapshot on a Droplet
2. Wait for snapshot to finish
3. Fetch snapshot action detail via API

When making request directly to the action: GET https://api.digitalocean.com/v2/actions/<action.id> the API returns data as expected:

{
    "action": {
        "id": "<action.id>",
        "status": "completed",
        "type": "snapshot",
        "started_at": "2022-04-27T00:07:13Z",
        "completed_at": "2022-04-27T00:15:40Z",
        "resource_id": "<droplet id>",

However, when action is fetched via droplet: GET https://api.digitalocean.com/v2/droplets/<droplet id>/actions/<action.id> completed_at field is wrong:

{
    "action": {
        "id": "<action.id>",
        "status": "completed",
        "type": "snapshot",
        "started_at": "2022-04-27T00:07:13Z",
        "completed_at": null, <== should not be null
        "resource_id": "<droplet id>",