Always treat headers as individual strings in pkg/http.

pkg/http/test/utils.dart

 // Copyright (c) 2012, 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.
 
 library test_utils;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:json' as json;
 import 'dart:uri';
@@ skipping 37 matching lines @@
     response.statusCode = 302;
     response.headers.set('location', serverUrl.resolve('/').toString());
     response.contentLength = 0;
     response.outputStream.close();
   });
 
   _server.defaultRequestHandler = (request, response) {
     consumeInputStream(request.inputStream).then((requestBodyBytes) {
       response.statusCode = 200;
       response.headers.contentType = new ContentType("application", "json");
+      response.headers.set('single', 'value');
 
       var requestBody;
       if (requestBodyBytes.isEmpty) {
         requestBody = null;
       } else if (request.headers.contentType.charset != null) {
         var encoding = requiredEncodingForCharset(
             request.headers.contentType.charset);
         requestBody = decodeString(requestBodyBytes, encoding);
       } else {
         requestBody = requestBodyBytes;
@@ skipping 115 matching lines @@
 const isSocketIOException = const _SocketIOException();
 
 /// A matcher for functions that throw SocketIOException.
 const Matcher throwsSocketIOException =
     const Throws(isSocketIOException);
 
 class _SocketIOException extends TypeMatcher {
   const _SocketIOException() : super("SocketIOException");
   bool matches(item, MatchState matchState) => item is SocketIOException;
 }