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.
Technically, there is no need to assign the status to a variable since it's not used in this `context` block.