`_signalLambdaEnd` request to Lambda extension should set `Content-Type`

In a user support request it was noted that they saw this error in the Node.js APM agent logs running in a Lambda:

```
{ "log.level": "error", "@timestamp": "2024-01-17T20:54:32.917Z", "log.logger": "elastic-apm-node", "ecs.version": "8.10.0", "event.module": "apmclient", "durationMs": 53.83446400007233,
"err": "unexpected response status code: 400",
"message": "_signalLambdaEnd error" }
```

and the matching APM server logs included:

```
{
"http.request.body.bytes": 0,
"image": "docker.elastic.co/cloud-assets/apm:7.17.10-0",
"http.request.method": "POST",
"log.level": "error",
"http.response.status_code": 400,
"user_agent.original": "apm-agent-nodejs/3.50.0 ...",
"service.name": "apm-server",
"message": "data validation error",
"url.original": "/intake/v2/events?flushed=true",
"error.message": "invalid content type: ''",
// ...
}
```

(Side note: This seems to be a misconfigured Lambda that has the APM agent talking directly to APM server, instead of via the Elastic Lambda extension. IOW, this log.error is likely to be very rare.)

In any case, adding a `Content-Type: application/json`, even though the request is empty should avoid this rare error log.