Until now, the vars matcher has unintentionally lacked parity with the
map directive: the destination placeholders of the map directive would
be expressed as placeholders, i.e. {foo}. But the vars matcher would
not use { }: vars foo value
This looked weird, and was confusing, since it implied that the key
could be dynamic, which doesn't seem helpful here.
I think this is a proper bug fix, since we're not used to accessing
placeholders literally without { } in the Caddyfile.
7c35bfa57c