DateOfBirth#to_s should always return a string

Author: chrismytton

lib/date_of_birth.rb

@@ -7,7 +7,7 @@ def initialize(date_string)
   end
 
   def to_s
-    return if match.nil?
+    return '' if match.nil?
     '%d-%02d-%02d' % [match[:year], month(match[:month]), match[:day]]
   end