[Forge] Fix IP forwarding for Forge 1.8 clients connecting to Spigot servers

This PR checks for and removes any null character seperated extra data from the initial handshake packet, restoring it to a vanilla state whilst it is being processed by Bungee. This allows for one of two outcomes:

* If IP forwarding is on, this extra data is not added to the Bungee <-> Server handshake, and the IP forwarding data is appended to the handshake instead. This allows Spigot servers to continue operating as intended with IP forwarding, but FML servers will not start a handshake with FML clients (which is fine for now, no FML 1.8 servers exist that allow for IP forwarding, as far as I can see).
* If IP forwarding is off, the extra data is re-appended to the handshake and sent to the server.

This works around issue #1277.