Merge pull request #15813 from DNNX/valid-action-name-refactoring

Relpace `=~ Regexp.new str` with `.include? str` in `_valid_action_name?`

actionpack/lib/abstract_controller/base.rb

@@ -255,7 +255,7 @@ module AbstractController
 
       # Checks if the action name is valid and returns false otherwise.
       def _valid_action_name?(action_name)
-        action_name !~ Regexp.new(File::SEPARATOR)
+        !action_name.to_s.include? File::SEPARATOR
       end
   end
 end