nghttp: prevent ContinueTimer double-invocation

If a 100 Continue interim response was received after the continue
timeout was reached, dispatch_continue() would force a double submission
of DATA frames. This patch prevents dispatch_continue() from doing
anything if the timer callback has already been invoked. This makes
ContinueTimer a single-shot mechanism, as originally intended.

src/nghttp.cc

@@ -344,7 +344,10 @@ void ContinueTimer::stop() {
 }
 
 void ContinueTimer::dispatch_continue() {
-  ev_feed_event(loop, &timer, 0);
+  // Only dispatch the timeout callback if it hasn't already been called.
+  if (ev_is_active(&timer)) {
+    ev_feed_event(loop, &timer, 0);
+  }
 }
 
 namespace {

src/nghttp.h

@@ -119,7 +119,8 @@ struct ContinueTimer {
   void start();
   void stop();
 
-  // Schedules an immediate run of the continue callback on the loop
+  // Schedules an immediate run of the continue callback on the loop, if the
+  // callback has not already been run
   void dispatch_continue();
 
   struct ev_loop *loop;