fix(fanout): commit request headers before fanout handoff (#1222)

Author: harmony7

runtime/fastly/builtins/fastly.cpp

@@ -243,6 +243,11 @@ bool Fastly::createFanoutHandoff(JSContext *cx, unsigned argc, JS::Value *vp) {
   }
   auto grip_upgrade_request = &request_value.toObject();
 
+  RootedObject request(cx, grip_upgrade_request);
+  if (!RequestOrResponse::commit_headers(cx, request)) {
+    return false;
+  }
+
   auto response_handle = host_api::HttpResp::make();
   if (auto *err = response_handle.to_err()) {
     HANDLE_ERROR(cx, *err);