Log response header from upstream

[Kamil Gorlo]

> Hyphen is used when variable isn't found.  Empty string - if it's
> found, but empty.
>

I see that variable defined by user is always empty (no hyphen) even
if it is not defined in location section connected with request (but
defined somewhere else in nginx config - other location section not
some section which is inherited). Is it normal?

Cheers,
Kamil Gorlo