Add test for the empty and non-existing varied-upon header resulting in individual cache objects
Add test for the empty and non-existing varied-upon header resulting in individual cache objects

Outcome of a discussion of the suggestion to treat empty headers like
non-existing for Vary processing:

https://www.varnish-cache.org/lists/pipermail/varnish-dev/2015-January/008131.html

chrismoulton/Varnish-Cache,ajasty-cavium/Varnish-Cache,mrhmouse/Varnish-Cache,chrismoulton/Varnish-Cache,gauthier-delacroix/Varnish-Cache,franciscovg/Varnish-Cache,feld/Varnish-Cache,ajasty-cavium/Varnish-Cache,varnish/Varnish-Cache,mrhmouse/Varnish-Cache,ajasty-cavium/Varnish-Cache,zhoualbeart/Varnish-Cache,feld/Varnish-Cache,gauthier-delacroix/Varnish-Cache,zhoualbeart/Varnish-Cache,franciscovg/Varnish-Cache,gauthier-delacroix/Varnish-Cache,gquintard/Varnish-Cache,zhoualbeart/Varnish-Cache,ajasty-cavium/Varnish-Cache,varnish/Varnish-Cache,chrismoulton/Varnish-Cache,franciscovg/Varnish-Cache,gauthier-delacroix/Varnish-Cache,mrhmouse/Varnish-Cache,franciscovg/Varnish-Cache,varnish/Varnish-Cache,gauthier-delacroix/Varnish-Cache,gquintard/Varnish-Cache,mrhmouse/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,chrismoulton/Varnish-Cache,chrismoulton/Varnish-Cache,zhoualbeart/Varnish-Cache,gquintard/Varnish-Cache,varnish/Varnish-Cache,ajasty-cavium/Varnish-Cache,franciscovg/Varnish-Cache,mrhmouse/Varnish-Cache,gquintard/Varnish-Cache,varnish/Varnish-Cache,feld/Varnish-Cache,zhoualbeart/Varnish-Cache