ask the named routes collection if the route is defined

we should not be accessing internals to figure out if a method is
defined.

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -94,6 +94,10 @@ module ActionDispatch
           @module  = Module.new
         end
 
+        def route_defined?(name)
+          @module.method_defined? name
+        end
+
         def helper_names
           @helpers.map(&:to_s)
         end

actionpack/lib/action_dispatch/testing/assertions/routing.rb

@@ -165,7 +165,7 @@ module ActionDispatch
 
       # ROUTES TODO: These assertions should really work in an integration context
       def method_missing(selector, *args, &block)
-        if defined?(@controller) && @controller && @routes && @routes.named_routes.helpers.include?(selector)
+        if defined?(@controller) && @controller && @routes && @routes.named_routes.route_defined?(selector)
           @controller.send(selector, *args, &block)
         else
           super