Adding X-Robots-Tag to http::header

So I'm trying to add an additional iRule to my VS that adds the "X-Robots-Tag" "noindex" to all responses.

What I have is a single iRule that contains the following code:

when HTTP_RESPONSE {

  HTTP::header insert "X-Robots-Tag" "noindex"

}

So that way I can just add this to any VS that needs it. However, when I do, and test using curl, I don't ever see the actual header get inserted. I'm obviously missing something but I haven't quite figured out what I'm missing.