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.
When I am testing a class (model or PORO), I always seperate the test using describes to group the tests for each method. I use contexts when I am testing the file as a whole and I need to seperate the tests into their own groups. I also use contexts inside describes when the method is complex.