Fix for external generators extend Erb::Generators

HAML and probably other generators extend this class and invoke
filename_with_extensions with the old signature (without format).
This makes the second argument optional and defaults it to the #format
method which could be overridden as well.

Closes #14132.

railties/lib/rails/generators/erb.rb

@@ -17,7 +17,7 @@ def handler
         :erb
       end
 
-      def filename_with_extensions(name, format)
+      def filename_with_extensions(name, format = self.format)
         [name, format, handler].compact.join(".")
       end
     end