Add a carriage return after the payload

When the payload doesn't have a newline character at the end, ElasticSearchOutput plugin returns this error:

    Plugin 'ElasticSearchOutput' error: HTTP response error status: 500 Internal Server Error

Tracing down it, leads me to discover that this was the error returned by ES:
```json
{
    "error": "ActionRequestValidationException[Validation Failed: 1: no requests added;]",
    "status": 500
}
```
That indicates an error that turned out to be that, I need one last newline character at the end of my json.

Also: `100% tests passed, 0 tests failed out of 23`

If other users, even the tests have this newline at the end, doesn't cause any trouble to always add one, and workarounds the other issue, that other users like me, could have.