Fix to_json / serializable hash issue when using two uploaders on the same model. 

1. Added two failing tests that illustrate the problem.
2. Fixed issue by changing serializable hash to use column.to_sym instead of string interpolation (which overwrites the method definition at the time mount_uploader is called)
3. Fix an unrelated issue that caused my tests to fail: check that options are defined before sending :[] (which nil doesn't repond to)