fix(server): reschedule keep alive interval timer once a pong is received

`KeepAliveState` did not transition from `PingSent` to `Scheduled` after
a pong was received. This prevented more than one ping to be sent by the
server. This fix checks if `ping_sent_at` has already been cleared by
`Ponger::poll` when `KeepAliveState::PingSent` state is active.

Fixes hyperium#2310

Author: pdcalado

src/proto/h2/ping.rs

@@ -442,7 +442,16 @@ impl KeepAlive {
                 let interval = shared.last_read_at() + self.interval;
                 self.timer.reset(interval);
             }
-            KeepAliveState::Scheduled | KeepAliveState::PingSent => (),
+            KeepAliveState::PingSent => {
+                if shared.is_ping_sent() {
+                    return;
+                }
+
+                self.state = KeepAliveState::Scheduled;
+                let interval = shared.last_read_at() + self.interval;
+                self.timer.reset(interval);
+            }
+            KeepAliveState::Scheduled => (),
         }
     }