Output payload ignored in "critical" TTL update

Ouput payload silently ignored when passed in "critical" TTL update API call.
```
put "/agent/check/update/:check_id"
with payload:
{
"Status": "critical",
"Ouptu": "This is critical error"
}
```

Result with empty Output section in consul health checks UI. 'passing' and 'warning' works as expected in docs.

If there is previous update with ouput, then it will be displayed in "critical" check after TTL expired as:
```
TTL expired (last output before timeout follows): PREV PASSING OR WARN OUTPUT
```

Start by reproducing the request to PUT /agent/check/update/:check_id with a critical status and output payload, then compare it with passing and warning updates. Trace the TTL health-check update path and verify that the critical check preserves and displays the supplied output, including after TTL expiration.

Written by the indexing model from the issue text.