Partially revert 0225a1

Commit 0225a1 prevented accessing a possibly null/undefined dictionary inside
the scope, but the way it was handling it was preventing formly-dynamic-name-update
listener from been registered.

The proposed solution keeps checking for if $scope.formOnParentScope is defined,
but inside the event handler, making it aware of scope changes
Merge pull request #99 from idbaigorria/fix-0225a1

Partially revert 0225a1