Fix null response handling.

lib/shelf_io.dart

Index: lib/shelf_io.dart
diff --git a/lib/shelf_io.dart b/lib/shelf_io.dart
index 301050f28713f5a4668516c287d02168be74887f..d5eb01760e564752ae492177848b48b6aff4d1ac 100644
--- a/lib/shelf_io.dart
+++ b/lib/shelf_io.dart
@@ -84,7 +84,8 @@ Future handleRequest(HttpRequest request, Handler handler) {
     return _logError('Error thrown by handler.\n$error', stackTrace);
   }).then((response) {
     if (response == null) {
-      response = _logError('null response from handler.');
+      return _writeResponse(
+          _logError('null response from handler.'), request.response);
     } else if (shelfRequest.canHijack) {
       return _writeResponse(response, request.response);
     }