For the next person who wants to do this (you cant…)
	<neek> how can one do a proxy_set_header inside an if?
	<neek> apparently that's not allowed.  because no one ever wants to
	          conditionally set a header to the upstream?
	<mfjordvald> neek: IT's a technical limitation, not a "no one would ever need
	             this" limitation.
	<neek> boggle.
	<neek> I can do it in apache, varnish, pound….
	<mfjordvald> Then use one of those.