[#4105] Create proxy method `headLink()`

- Creates a proxy "headLink" method which allows us usage as described
  in the manual. Proxies to `__invoke`.
Merge branch 'hotfix/4105' of https://github.com/weierophinney/zf2 into weierophinney-hotfix/4105
Merge branch 'weierophinney-hotfix/4105'