Update notification.py

self.data is now correctly formatted, previously it would cause an invalid message when there is an error.

Example:
Previously:
{
    "exabgp": "3.4.0",
    "time": 1402495294,
    "id": "producer.local_5238_192.168.33.9",
    "counter": 2,
    "neighbor": {
        "ip": "192.168.33.9",
        "state": "down",
        "type": "state",
        "reason": "out loop reset notification received (2,3) OPEN message error / Bad BGP Identifier./ ??!	"
    }
}

With the modification:
{
    "exabgp": "3.4.0",
    "time": 1402496640,
    "id": "producer.local_5238_192.168.33.9",
    "counter": 2,
    "neighbor": {
        "ip": "192.168.33.9",
        "state": "down",
        "type": "state",
        "reason": "out loop reset notification received (2,3) OPEN message error / Bad BGP Identifier./ C0A82109"
    }
}