fix soft_validations for required fields #4315

dm-core AND dm-validations checks for :required => true fields. The
dm-core check cannot be disabled using save! so this disables it.

This also adds preliminary specs for soft_validations.
use rspec predicate matchers #4315
Merge pull request #6 from NUBIC/4315_disable_validations_option

fix soft_validations for required fields #4315