Update msgpack_packet.py

Using together with non-python implementations causes issues as the payload is optional.

Currently:
`self.data = decoded['data']`

Should be:
`self.data = decoded.get('data')`

Reference:
d556a0ec4d/src/socketio/msgpack_packet.py (L16)

Documentation: 
https://github.com/socketio/socket.io-protocol#packet-format

src/socketio/msgpack_packet.py

@@ -13,6 +13,6 @@ class MsgPackPacket(packet.Packet):
         """Decode a transmitted package."""
         decoded = msgpack.loads(encoded_packet)
         self.packet_type = decoded['type']
-        self.data = decoded['data']
+        self.data = decoded.get('data')
         self.id = decoded.get('id')
         self.namespace = decoded['nsp']