Use date objects when building office residences

Since the interface to select this date uses the classic multi-field
interface for day, month and year, we're transforming it into a date in
the Office::Residence initializer.

However, the factory to build an office residence does not assign the
paremeters in the initializer but using the `date_of_birth=` method, so
when doing so we need to use a date instead of a string.

spec/models/officing/residence_spec.rb

@@ -38,7 +38,7 @@ describe Officing::Residence do
       let(:custom_residence) do
         build(:officing_residence,
               document_number: "12345678Z",
-              date_of_birth: "01/01/1980",
+              date_of_birth: Date.parse("01/01/1980"),
               postal_code: "28001")
       end