fixes #2776 - smart class parameter override fails validation

validate_and_cast_value in lookup_value.rb is being called twice.  In the second run, the yaml is already cast to a native data type.  YAML.load(YAML.load(value)) fails of course.

I suspect the culprit is Rails calling before_validation twice?

This makes sure we don't try to cast a native data type other than String here, which fixes the problem.