Retrieve underlying net::UdpSocket from UdpSocket

[Technici4n]

With this PR, it is now possible to retrieve a UdpSocket's underlying net::UdpSocket.

I need this because the default socket buffer size is too small for my UDP usage. The size can be easily changed using the crate net2, but therefore it needs access to the net::UdpSocket.

Thanks in advance!

[coveralls]

Changes Unknown when pulling 604e37f57cdaba5193664365b2eb1740c8df6ea3 on Technici4n:master into ** on BonsaiDen:master**.