Do not reset $id if found

- Pointed out by andrer in #zftalk.dev
Write test for HEAD behavior with identifier

- Wrote test to prove that HEAD is being called with identifier when
  found; failed before change, and works afterwards
Merge branch 'hotfix/restful-head-identifier' of https://github.com/weierophinney/zf2 into weierophinney-hotfix/restful-head-identifier
Merge branch 'weierophinney-hotfix/restful-head-identifier'