Implement TCPKeepAliveListener for plaintext sockets as well

[joonas-fi]

This removed line documented that TCPKeepAliveListener should've been used with the plaintext socket as well: https://github.com/mailgun/manners/commit/448d7b595e3f90682487e112905a38b19f22fbb7#diff-804732da4aad8a84ebedfb545342f187L60

I've now added it to the plaintext socket, because I hit Azure's disgusting "disconnect sockets with inactivity for 4 minutes" feature here, when fronted by Cloudflare: https://github.com/containous/traefik/issues/1046