HTTP/2: improved handling of "keepalive_timeout 0".

Without explicit handling, a zero timer was actually added, leading to
multiple unneeded syscalls.  Further, sending GOAWAY frame early might
be beneficial for clients.

Reported by Sergey Kandaurov.

src/http/v2/ngx_http_v2.c

diff --git a/src/http/v2/ngx_http_v2.c b/src/http/v2/ngx_http_v2.c
index 005db0b..8563205 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -1368,7 +1368,9 @@
     clcf = ngx_http_get_module_loc_conf(h2c->http_connection->conf_ctx,
                                         ngx_http_core_module);
 
-    if (h2c->connection->requests >= clcf->keepalive_requests) {
+    if (clcf->keepalive_timeout == 0
+        || h2c->connection->requests >= clcf->keepalive_requests)
+    {
         h2c->goaway = 1;
 
         if (ngx_http_v2_send_goaway(h2c, NGX_HTTP_V2_NO_ERROR) == NGX_ERROR) {