* caddyhttp: Enhance vars matcher
Enable "or" logic for multiple values.
Fall back to checking placeholders if not a var name.
* Fix tests (thanks @mohammed90 !)
* add integration test for null header matcher
* implement null header matcher syntax
* avoid repeating magic !
* check for field following ! character
* caddyfile: support vars and vars_regexp matchers in the caddyfile
* caddyfile: matchers: Brian Kernighan said printf is good debugging tool but didn't say keep them around
