Expose HTTP/2 ping

[AndrewBarba]

This would solve...

Some http2 servers expect to receive PING frame during long periods of inactivity. For example, Apple's APNS service requires sending a PING in order to keep a connection alive. Note: keep alive is not enough.

The implementation should look like...

Both Pool and Client should expose a ping() method which calls the underlying Http2Session.ping() method.

Real-world issue

AndrewBarba/apns2#97

Real-world work around

AndrewBarba/apns2#98

[mcollina]

I think for HTTP/2 connections we could send a ping() at a given interval instead. Would that work for you? In case, would you like to send a PR?

[AndrewBarba]

@mcollina yes an interval option would be perfect. I'll take a look at implementing