Ignore Web browsers "Preview" http request #3285

Reported in forum

Proposal from Anthon:

  We just need to exclude visits with the X-Moz: prefetch (Mozilla), X-Purpose: preview (Safari, Opera), and X-Purpose: instant (Chrome) ```

something like

$_SERVER["HTTP_X_PURPOSE"] == "preview" || $_SERVER['HTTP_X_MOZ'] == "prefetech")

I would have added a UA check as an additional guard against header spoofing.

Also, what about "X-Purpose: instant"?

p.s. is the commit hook not working?

Did you see any doc on X-purpose: instant?

Header spoofing: that's fine with me if users spoof this header to exclude themselves from tracking? :)

(In [6638]) Refs #3285

https://developers.google.com/search?q=%22x-purpose%22

For Chrome, it was "preview" at one point, and then later changed to "instant". Oh, and at one point, it seems there was also a bug (now fixed) where Chrome sent: "X-Purpose: : instant" (extra colon and space).