That's great work @girish - it's not often you step out for the day and return to see your post "solved". I was going to include that it should be a simple nginx config thing but I know better. And it's unfortunate the request caching is a nightmare to enforce "must-staple".
Regardless, the win here is that you have it enabled. That should now be reflected in test reults from Mozilla Observatory (extended to Qualys SSL Labs & ImmuniWeb). Thanks for putting in the grunts on this one.
W