allow options to concat so validate can be turned off

I needed to turn off the validations on the related items when using concat but I was unable to do it. I've dig in the code and I noticed with a slight change we can pass `validate: false` to the `save` method from concat.
I am not too familiar with rspec and not sure how to write a test for this. Can someone give me some pointers? Maybe and example spec for a similar feature?
Thanks