Fix shelf_web_socket's Connection handling.

pkg/shelf_web_socket/lib/src/web_socket_handler.dart

Index: pkg/shelf_web_socket/lib/src/web_socket_handler.dart
diff --git a/pkg/shelf_web_socket/lib/src/web_socket_handler.dart b/pkg/shelf_web_socket/lib/src/web_socket_handler.dart
index ac5786a95cadb14f5518c0e35634a33ecb7df75f..7ab8e7eb9764b20c8a77453ba3b9b2c5b8646afd 100644
--- a/pkg/shelf_web_socket/lib/src/web_socket_handler.dart
+++ b/pkg/shelf_web_socket/lib/src/web_socket_handler.dart
@@ -28,7 +28,9 @@ class WebSocketHandler {
 
     var connection = request.headers['Connection'];
     if (connection == null) return _notFound();
-    if (connection.toLowerCase() != 'upgrade') return _notFound();
+    var tokens = connection.toLowerCase().split(',')
+        .map((token) => token.trim());
+    if (!tokens.contains('upgrade')) return _notFound();
 
     var upgrade = request.headers['Upgrade'];
     if (upgrade == null) return _notFound();