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.