package:appengine v0.13.13

• Rename error classes to use the Exception suffix
  (AppEngineException, NetworkException, ProtocolException,
  ServiceException, ApplicationException) to follow Dart conventions for
  types implementing Exception. The old *Error names remain available as
  deprecated type aliases, so this change is non-breaking.
• Automated formatting and fixes.
• Cleanup legacy code.
• Fix X-Forwarded-For handling.