Dob validation

I wrote rspec tests when I submitted the pr for dob_validation. I can remember writing them. Somehow, though, they didn't come through to the pr. I've long since deleted the branch so can't see what went wrong. 
Here is a small pr to put the tests back in to the model specs.
@NikitaAvvakumov 
I don't want to sound flippant but I don't give it that much thought. describe and context are synonyms and, for me, both are clear. Having said that if it makes it clearer to other people I don't object to changing it.