Fixed keepalive JSON object

The keepalive message is invalid with the current keepalive encoding method, with this pull request it will be fixed.

Previous:
```Text
{
    "exabgp": "3.4.0",
    "time": 1405934239,
    "host": "producer.local",
    "pid": "14590",
    "ppid": "14460",
    "counter": 3,
    "type": "keepalive",
    "header": "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF001304",
    "neighbor": {
        "ip": "192.168.33.12",
        
    }
}
```

Now:
```Text
{
    "exabgp": "3.4.0",
    "time": 1405936047,
    "host": "producer.local",
    "pid": "15171",
    "ppid": "14460",
    "counter": 4,
    "type": "keepalive",
    "header": "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF001304",
    "neighbor": {
        "ip:": "192.168.33.12"
    }
}
```