Always treat headers as individual strings in pkg/http.

pkg/http/test/client_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.
 
 library client_test;
 
 import 'dart:io';
 import 'dart:uri';
 
 import 'package:unittest/unittest.dart';
 import 'package:http/http.dart' as http;
 import 'package:http/src/utils.dart';
 import 'utils.dart';
 
 void main() {
   setUp(startServer);
   tearDown(stopServer);
 
   test('#send a StreamedRequest', () {
     var client = new http.Client();
     var request = new http.StreamedRequest("POST", serverUrl);
     request.headers[HttpHeaders.CONTENT_TYPE] =
       'application/json; charset=utf-8';
 
     expect(client.send(request).then((response) {
       expect(response.request, equals(request));
       expect(response.statusCode, equals(200));
+      expect(response.headers['single'], equals('value'));
+      // dart:io internally normalizes outgoing headers so that they never have
+      // multiple headers with the same name, so there's no way to test whether
+      // we handle that case correctly.
+
       return response.stream.bytesToString();
     }).whenComplete(client.close), completion(parse(equals({
       'method': 'POST',
       'path': '/',
       'headers': {
         'content-type': ['application/json; charset=utf-8'],
         'transfer-encoding': ['chunked']
       },
       'body': '{"hello": "world"}'
     }))));
 
     request.sink.add('{"hello": "world"}'.charCodes);
     request.sink.close();
   });
 
   test('#send with an invalid URL', () {
     var client = new http.Client();
     var url = Uri.parse('http://http.invalid');
     var request = new http.StreamedRequest("POST", url);
     request.headers[HttpHeaders.CONTENT_TYPE] =
         'application/json; charset=utf-8';
 
     expect(client.send(request), throwsSocketIOException);
 
     request.sink.add('{"hello": "world"}'.charCodes);
     request.sink.close();
   });
 }