Mark parent category menu active on single custom post view in WordPress

Recently I developed a theme for one of my client and I had to highlight the menu item of parent category in main menu when one of its associated single custom post is viewed. For that I had to add an action in my functions.php file for nav_menu_css_class. What is does is it returns the ‘active’ class which WordPress adds to the parent category menu item in the main menu. You can see the code at Gist here.