fix(rxModalAction): no longer destroys scope.fields on modal submit

BREAKING CHANGE: 'fields' property no longer checks if scope.fields is already defined.
If you're defining 'fields' information inside the modal pre-hook, everything should work as normal
If you're defining it before the pre-hook (on page load), this will break. You will need to move that definition into a pre-hook function

Fix for https://github.com/rackerlabs/chunky_bacon/pull/203#issuecomment-40252691