Ignore SignalException and Rack instrumentation fixes

Changed

• Send check-ins concurrently. When calling Appsignal::CheckIn.cron, instead of blocking the current thread while the check-in events are sent, schedule them to be sent in a separate thread.

  When shutting down your application manually, call Appsignal.stop to block until all scheduled check-ins have been sent.

Fixed

• Make our Rack BodyWrapper behave like a Rack BodyProxy. If a method doesn't exist on our BodyWrapper class, but it does exist on the body, behave like the Rack BodyProxy and call the method on the wrapped body.
• Do not report SignalException errors from our at_exit error reporter.

View the Ruby gem v4.0.4 changelog for more information.