Exposed embedded key existence in HalResource, added a test to check …

[weierophinney]

Provide a narrative description of what you are trying to accomplish:

• [x] Are you fixing a bug?
  • [x] Detail how the bug is invoked currently.
  • [x] Detail the original, incorrect behavior.
  • [x] Detail the new, expected behavior.
  • [x] Base your feature on the master branch, and submit against that branch.
  • [x] Add a regression test that demonstrates the bug, and proves the fix.
  • [ ] Add a CHANGELOG.md entry for the fix.

See https://github.com/zendframework/zend-expressive-hal/issues/52 for details

---

Originally posted by @jguittard at https://github.com/zendframework/zend-expressive-hal/pull/53