Make date input field name gsub indepedent of select box order

Date field label target may be tied to any of the day, month, year
select box inputs; for example, simple_form chooses the first:
https://github.com/plataformatec/simple_form/blob/6f2a61015098d43baac67a5f64c5fb4ba9031426/lib/simple_form/inputs/date_time_input.rb#L20

This minor change ensures the matcher will parse the correct field
name regardless of the order.