Fix #dup with custom serialisation

While `write_uploader` is aliased to `write_attribute` by default,
it may be overridden in the model. This change ensures that the
override method is called when clearing the attribute.

lib/carrierwave/orm/activerecord.rb

@@ -51,7 +51,7 @@ def initialize_dup(other)
           super
           @_mounters[:"#{column}"] = nil
           # The attribute needs to be cleared to prevent it from picked up as identifier
-          write_attribute(_mounter(:#{column}).serialization_column, nil)
+          write_uploader(_mounter(:#{column}).serialization_column, nil)
           _mounter(:"#{column}").cache(old_uploaders)
         end
 

spec/orm/activerecord_spec.rb

@@ -2181,6 +2181,30 @@ def initialize_dup(*)
       end
     end
 
+    context 'when #write_uploader is overridden in the model' do
+      before do
+        Event.class_eval do
+          attribute :called_write_uploader_with, :json, default: {}
+          def write_uploader(*args)
+            self.called_write_uploader_with[args] ||= 0
+            self.called_write_uploader_with[args] += 1
+            write_attribute(*args)
+          end
+        end
+        @event = Event.new
+      end
+
+      it "clears previous attribute value to prevent unintended deduplication" do
+        @event.image = stub_file('test.jpeg')
+        @event.save
+        new_event = @event.dup
+        new_event.save
+
+        expect(new_event.called_write_uploader_with[[:image, nil].to_s]).to eq 1
+        expect(new_event.image.url).to eq '/uploads/test.jpeg'
+      end
+    end
+
     context ':mount_on in options' do
       before { Event.mount_uploader(:image_v2, @uploader, mount_on: :image) }
       it do