HTTP/2: refuse streams with data until SETTINGS is acknowledged.

A client is allowed to send requests before receiving and acknowledging
the SETTINGS frame.  Such a client having a wrong idea about the stream's
could send the request body that nginx isn't ready to process.

The previous behavior was to send RST_STREAM with FLOW_CONTROL_ERROR in
such case, but it didn't allow retrying requests that have been rejected.

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 a207075..7475497 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -1058,6 +1058,12 @@
         goto rst_stream;
     }
 
+    if (!h2c->settings_ack && !(h2c->state.flags & NGX_HTTP_V2_END_STREAM_FLAG))
+    {
+        status = NGX_HTTP_V2_REFUSED_STREAM;
+        goto rst_stream;
+    }
+
     node = ngx_http_v2_get_node_by_id(h2c, h2c->state.sid, 1);
 
     if (node == NULL) {
@@ -1878,7 +1884,7 @@
             return ngx_http_v2_connection_error(h2c, NGX_HTTP_V2_SIZE_ERROR);
         }
 
-        /* TODO settings acknowledged */
+        h2c->settings_ack = 1;
 
         return ngx_http_v2_state_complete(h2c, pos, end);
     }

src/http/v2/ngx_http_v2.h

diff --git a/src/http/v2/ngx_http_v2.h b/src/http/v2/ngx_http_v2.h
index 7a77c27..1adf8de 100644
--- a/src/http/v2/ngx_http_v2.h
+++ b/src/http/v2/ngx_http_v2.h
@@ -141,6 +141,7 @@
     ngx_uint_t                       last_sid;
 
     unsigned                         closed_nodes:8;
+    unsigned                         settings_ack:1;
     unsigned                         blocked:1;
 };