search docs/test/fix tapname-search

This is the promised followup to #2496
- docs
- test
- bugfix: search string was being tested against fully-qualified
  Cask name including Tap, so "brew cask search phinze" would
  (confusingly) return all Casks.
Merge pull request #2545 from rolandwalker/search_followups

search docs/test/fix tapname-search