Actions API does not return `completed_at` param

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/`
the API returns data as expected:
```json
{
"action": {
"id": "",
"status": "completed",
"type": "snapshot",
"started_at": "2022-04-27T00:07:13Z",
"completed_at": "2022-04-27T00:15:40Z",
"resource_id": "",
```
However, when action is fetched via droplet:
GET `https://api.digitalocean.com/v2/droplets//actions/`
`completed_at` field is wrong:
```json
{
"action": {
"id": "",
"status": "completed",
"type": "snapshot",
"started_at": "2022-04-27T00:07:13Z",
"completed_at": null, <== should not be null
"resource_id": "",
```