Remove unnecessary to_s from try_type_from_number

Our CARD_TYPES hash now uses strings as keys so we no longer need to
stringify them.

core/app/models/spree/credit_card.rb

@@ -106,7 +106,7 @@ def set_last_digits
     #   number, otherwise the empty string
     def try_type_from_number
       CARD_TYPES.each do |type, pattern|
-        return type.to_s if number =~ pattern
+        return type if number =~ pattern
       end
       ''
     end