enable unit testing of EssentialAction (issue #2011)

An EssentialAction can be tested without a running
FakeApplication.

Check that the EssentialAction uses the classloader of
the running application if any.
Merge pull request #2024 from yanns/issue-2011

Enable unit testing of EssentialAction (issue #2011) by allowing an EssentialAction to run without an installed Application
Merge remote-tracking branch: upstream/master