fix process request when a response already closed (#550)

src/kemal/route_handler.cr

@@ -45,6 +45,7 @@ module Kemal
     # Processes the route if it's a match. Otherwise renders 404.
     private def process_request(context)
       raise Kemal::Exceptions::RouteNotFound.new(context) unless context.route_found?
+      return if context.response.closed?
       content = context.route.handler.call(context)
 
       if !Kemal.config.error_handlers.empty? && Kemal.config.error_handlers.has_key?(context.response.status_code)