Always treat headers as individual strings in pkg/http.

pkg/http/test/client_test.dart

Index: pkg/http/test/client_test.dart
diff --git a/pkg/http/test/client_test.dart b/pkg/http/test/client_test.dart
index e29cef5a341bec215c0a49d446b2a143b1342561..fd056c9c4233b18b8f8571bff339d73751ebe9ad 100644
--- a/pkg/http/test/client_test.dart
+++ b/pkg/http/test/client_test.dart
@@ -25,6 +25,11 @@ void main() {
     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',