pkg/shelf - send Date header in shelf_io

pkg/shelf/lib/shelf_io.dart

Index: pkg/shelf/lib/shelf_io.dart
diff --git a/pkg/shelf/lib/shelf_io.dart b/pkg/shelf/lib/shelf_io.dart
index d8d4d9ca3175e9b0cfea38c36302eeee00bfeb22..6582e5a0688658b38c628629981dfb47bb9ae35d 100644
--- a/pkg/shelf/lib/shelf_io.dart
+++ b/pkg/shelf/lib/shelf_io.dart
@@ -121,6 +121,11 @@ Future _writeResponse(Response response, HttpResponse httpResponse) {
     var value = httpResponse.headers.value(HttpHeaders.SERVER);
     httpResponse.headers.set(HttpHeaders.SERVER, '$value with Shelf');
   }
+
+  if (!response.headers.containsKey(HttpHeaders.DATE)) {
+    httpResponse.headers.date = new DateTime.now().toUtc();
+  }
+
   return httpResponse.addStream(response.read())
       .then((_) => httpResponse.close());
 }