fix endless loop on flatten method

temporarily fixes https://github.com/thedersen/backbone.validation/pull/133 'Only flatten validated attributes' : filtering on validated attributes but considers also an attrs option containing specific attributes to validate in order to propagate attrs options from validate method.

This fixes #130 'endless loop on flatten method' while keeping full validation capacities.

This fix needs some complementary tests but is contributed ti fix quickly some blocking bugs. Once fully tested should be contributed to https://github.com/thedersen/backbone.validation

This PR also updates resthub-validation tests to follow Backbone changes since 1.0.0 (set does not trigger validation anymore)