Refactor: GTIN::Base.prioritize_before does not touch GTIN internals

Manipulating `GTIN.prioritized_gtin_classes` now only happens through
helper methods on GTIN, theoretically allowing the list itself to become
private.

lib/barcodevalidation/gtin.rb

@@ -22,7 +22,7 @@ def prioritized_gtin_classes
 
       # Adds the provided class to the back of the list of prioritized GTIN classes.
       def append_gtin_class(gtin_class)
-        return if prioritized_gtin_classes.include?(gtin_class)
+        return if gtin_class?(gtin_class)
 
         prioritized_gtin_classes.push(gtin_class)
         nil
@@ -34,6 +34,21 @@ def remove_gtin_class(gtin_class)
         nil
       end
 
+      # Is this a registered prioritized GTIN class?
+      # @return [true, false]
+      def gtin_class?(gtin_class)
+        prioritized_gtin_classes.include?(gtin_class)
+      end
+
+      # @param [Class] high_priority_class The higher priority GTIN class you want to move before the low priority class
+      # @param [Class] low_priority_class The low priority GTIN class that the high priority one is moved before
+      def reprioritize_before(high_priority_class, low_priority_class)
+        low_priority_index = prioritized_gtin_classes.index(low_priority_class)
+        remove_gtin_class(high_priority_class)
+        prioritized_gtin_classes.insert(low_priority_index, high_priority_class)
+        nil
+      end
+
       private
 
       def class_for_input(input)

lib/barcodevalidation/gtin/base.rb

@@ -34,13 +34,9 @@ def self.inherited(subclass)
 
       # Ensure this class is earlier in the GTIN classes list than +other_gtin_class+ and thus will get asked earlier if it handles a GTIN.
       def self.prioritize_before(other_gtin_class)
-        other_index = BarcodeValidation::GTIN.prioritized_gtin_classes.index(other_gtin_class)
-        raise ArgumentError, "The class you want to prioritize before is not a registered prioritized GTIN class." if other_index.nil?
+        raise ArgumentError, "The class you want to prioritize before is not a registered prioritized GTIN class." unless GTIN.gtin_class?(other_gtin_class)
 
-        BarcodeValidation::GTIN.remove_gtin_class(self)
-        BarcodeValidation::GTIN.prioritized_gtin_classes.insert(other_index, self)
-
-        nil
+        GTIN.reprioritize_before(self, other_gtin_class)
       end
 
       # This class is abstract and should not be included in the list of GTIN classes that actually implement a GTIN.