Fix broken Director auth headers and add regression test

[jhiemstrawisc]

Since the bug isn't super easy to catch in the diff, it was coming from:

cacheNS := NamespaceAd{}
cacheNS.Path = ns.Path
cacheNS.RequireToken = ns.UseTokenOnRead
cacheAdMap[cacheAd] = append(cacheAdMap[cacheAd], cacheNS)

I'm not sure why I ever did this because the appended value (cacheNS) should have just been nsAd like it is for the similar origin logic. I'll probably figure it out around the same time I figure out how many licks it takes to get to the center of a tootsie pop.

Once we're comfortable merging this, I'll patch it into 7.1 as this constitutes a bugfix.

Fixes issue #258

[turetske]

LGTM