nghttpx: Add x-http2-push header field for pushed resource

Fixes GH-352

src/shrpx_http2_upstream.cc

@@ -1321,8 +1321,9 @@ int Http2Upstream::on_downstream_header_complete(Downstream *downstream) {
 
   size_t nheader = downstream->get_response_headers().size();
   auto nva = std::vector<nghttp2_nv>();
-  // 3 means :status and possible server and via header field.
-  nva.reserve(nheader + 3 + get_config()->add_response_headers.size());
+  // 4 means :status and possible server, via and x-http2-push header
+  // field.
+  nva.reserve(nheader + 4 + get_config()->add_response_headers.size());
   std::string via_value;
   auto response_status = util::utos(downstream->get_response_http_status());
   nva.push_back(http2::make_nv_ls(":status", response_status));
@@ -1376,6 +1377,12 @@ int Http2Upstream::on_downstream_header_complete(Downstream *downstream) {
     nva.push_back(http2::make_nv(p.first, p.second));
   }
 
+  if (downstream->get_stream_id() % 2 == 0) {
+    // This header field is basically for human on client side to
+    // figure out that the resource is pushed.
+    nva.push_back(http2::make_nv_ll("x-http2-push", "1"));
+  }
+
   if (LOG_ENABLED(INFO)) {
     log_response_headers(downstream, nva);
   }