[BUGFIX] Allow keyboard events to work with the action helper.

The way action helper was validating the modifier keys against the
helper's allowedKeys option assumed a mouse/touch event. This commit
changes that to treat mouse/touch events differently.

Fixes #3340
[DOC release] Add Ember.Application.testHelper documentation.
[BUGFIX beta] Add spec for unhandled exception being logged by Test.adapter.exception

The changes in exception handling introduced in
cf0caa04f625067ec3b4629acdec12c8517df94e made a test assertion in an
existing test obsolete, since it wasn't called anymore.

This commit adds a spec for Test.adapter.exception being invoked, when
an exception is not handled.