Add ping to WebSocket example feed.pl

[preaction]

@jberger says if we add a regular ping, we can be better-notified when connections go down for strange reasons. Mojolicious will automatically respond to pings in Mojo::UserAgent, so we just need to send them. This should make the WebSocket APIs more reliable.

[jberger]

Sending a ping frame is the frame example shown for the Controller's send method. Note for clarity that a frame is sent by passing an arrayref to send.