Allow an empty HTTP resason phrase

tests/standalone/io/regress_9194_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:io';
+
+void main() {
+  HttpServer.bind('127.0.0.1', 0).then((server) {
+      server.listen((request) {
+        request.response
+            ..reasonPhrase = ''
+            ..close();
+      });
+
+      HttpClient client = new HttpClient();
+      client.get("127.0.0.1", server.port, "/")
+          .then((HttpClientRequest request) {
+            return request.close();
+          })
+          .then((HttpClientResponse response) {
+            Expect.equals("", response.reasonPhrase);
+            server.close();
+            client.close();
+          });
+    });
+}