Define methods on dynamic module

Currently you cannot override the generated methods and access super ie:

```ruby
monetize :price_cents

def price=(value)
   # stuff
   super # NoMethodError
end
```

This happens because the method is defined on the class directly so their is no super method to call. A solution today is to create an alias (`alias :old_price= :price=`) before overriding the method as is recommending in issue #507.

Instead, a new module can be created, included into the class, and the methods defined on it allowing `super` to work as expected.

lib/money-rails/active_record/monetizable.rb

@@ -9,6 +9,8 @@ class ReadOnlyCurrencyException < MoneyRails::Error; end
       extend ActiveSupport::Concern
 
       module ClassMethods
+        MODULE_NAME = :DynamicMoneyAttributes
+
         def monetized_attributes
           monetized_attributes = @monetized_attributes || {}.with_indifferent_access
 
@@ -116,23 +118,31 @@ def monetize(*fields)
               end
             end
 
-
-            # Getter for monetized attribute
-            define_method name do |*args, **kwargs|
-              read_monetized name, subunit_name, options, *args, **kwargs
+            if const_defined?(MODULE_NAME, false)
+              mod = const_get(MODULE_NAME)
+            else
+              mod = const_set(MODULE_NAME, Module.new)
+              include mod
             end
 
-            # Setter for monetized attribute
-            define_method "#{name}=" do |value|
-              write_monetized name, subunit_name, value, validation_enabled, instance_currency_name, options
-            end
+            mod.module_eval do
+              # Getter for monetized attribute
+              define_method name do |*args, **kwargs|
+                read_monetized name, subunit_name, options, *args, **kwargs
+              end
+
+              # Setter for monetized attribute
+              define_method "#{name}=" do |value|
+                write_monetized name, subunit_name, value, validation_enabled, instance_currency_name, options
+              end
 
-            if validation_enabled
-              # Ensure that the before_type_cast value is cleared when setting
-              # the subunit value directly
-              define_method "#{subunit_name}=" do |value|
-                instance_variable_set "@#{name}_money_before_type_cast", nil
-                write_attribute(subunit_name, value)
+              if validation_enabled
+                # Ensure that the before_type_cast value is cleared when setting
+                # the subunit value directly
+                define_method "#{subunit_name}=" do |value|
+                  instance_variable_set "@#{name}_money_before_type_cast", nil
+                  write_attribute(subunit_name, value)
+                end
               end
             end
 

spec/active_record/monetizable_spec.rb

@@ -30,6 +30,23 @@ def update_product(*attributes)
 
       context ".monetized_attributes" do
 
+        it "allows adds methods to the inheritance chain" do
+          class MyProduct < ActiveRecord::Base
+            self.table_name = :products
+            monetize :price_cents
+            attr_reader :side_effect
+
+            def price=(value)
+              @side_effect = true
+              super
+            end
+          end
+
+          p = MyProduct.new(price: 10)
+          expect(p.price).to eq Money.new(10_00)
+          expect(p.side_effect).to be_truthy
+        end
+
         class InheritedMonetizeProduct < Product
           monetize :special_price_cents
         end