BUG/MINOR: proto_http: properly reset the stream's call rate on keep-alive

The stream's call rate measurement was added by commit 2e9c1d296 ("MINOR:
stream: measure and report a stream's call rate in "show sess"") but it
forgot to reset it in case of HTTP keep-alive (legacy mode), resulting
in incorrect measurements.

No backport is needed, unless the patch above is backported.
diff --git a/src/proto_http.c b/src/proto_http.c
index 82292c1..5776a47 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -3450,6 +3450,7 @@
 	s->task->cpu_time  = 0;
 	s->task->lat_time  = 0;
 	s->task->call_date = ((profiling & HA_PROF_TASKS_MASK) >= HA_PROF_TASKS_AUTO) ? now_mono_time() : 0;
+	s->call_rate.curr_sec = s->call_rate.curr_ctr = s->call_rate.prev_ctr = 0;
 
 	s->logs.accept_date = date; /* user-visible date for logging */
 	s->logs.tv_accept = now;  /* corrected date for internal use */