detach the BitmaskAttributes::Proxy instance variable on dup

Before modifying the bitmask of a dupped object would also modify the original
object. This commit resets the proxy before dupping it which fixes this issue.

lib/bitmask_attributes/definition.rb

@@ -40,9 +40,19 @@ def generate_bitmasks_on(model)
       end
 
       def override(model)
+        override_dup(model)
         override_getter_on(model)
         override_setter_on(model)
       end
+
+      def override_dup(model)
+        model.class_eval %(
+          def dup
+            @#{attribute} = nil
+            super
+          end
+        )
+      end
 
       def override_getter_on(model)
         model.class_eval %(