gRPC: WINDOW_UPDATE after END_STREAM handling (ticket #1797).

As per https://tools.ietf.org/html/rfc7540#section-6.9,
WINDOW_UPDATE received after a frame with the END_STREAM flag
should be handled and not treated as an error.

src/http/modules/ngx_http_grpc_module.c

diff --git a/src/http/modules/ngx_http_grpc_module.c b/src/http/modules/ngx_http_grpc_module.c
index 9e62d8e..992211e 100644
--- a/src/http/modules/ngx_http_grpc_module.c
+++ b/src/http/modules/ngx_http_grpc_module.c
@@ -2091,7 +2091,8 @@
             }
 
             if (ctx->stream_id && ctx->done
-                && ctx->type != NGX_HTTP_V2_RST_STREAM_FRAME)
+                && ctx->type != NGX_HTTP_V2_RST_STREAM_FRAME
+                && ctx->type != NGX_HTTP_V2_WINDOW_UPDATE_FRAME)
             {
                 ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
                               "upstream sent frame for closed stream %ui",