HTTP/2: write logs when refusing streams with data.

Refusing streams is known to be incorrectly handled at least by IE, Edge
and Safari.  Make sure to provide appropriate logging to simplify fixing
this in the affected browsers.

src/http/v2/ngx_http_v2.c

@@ -1060,6 +1060,10 @@ ngx_http_v2_state_headers(ngx_http_v2_connection_t *h2c, u_char *pos,
 
     if (!h2c->settings_ack && !(h2c->state.flags & NGX_HTTP_V2_END_STREAM_FLAG))
     {
+        ngx_log_error(NGX_LOG_INFO, h2c->connection->log, 0,
+                      "client sent stream with data "
+                      "before settings were acknowledged");
+
         status = NGX_HTTP_V2_REFUSED_STREAM;
         goto rst_stream;
     }