HTTP/2: fixed send timer handling.

Checking for return value of c->send_chain() isn't sufficient since there
are data can be left in the SSL buffer.  Now the wew->ready flag is used
instead.

In particular, this fixed a connection leak in cases when all streams were
closed, but there's still some data to be sent in the SSL buffer and the
client forgot about the connection.

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 959e3a6..0ce8bce 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -549,7 +549,7 @@
         c->tcp_nodelay = NGX_TCP_NODELAY_SET;
     }
 
-    if (cl) {
+    if (!wev->ready) {
         ngx_add_timer(wev, clcf->send_timeout);
 
     } else {