ChangingContext.Merged creates duplicate is_null attributes on any incoming property when a type attribute exists

[kentonvk]

When using the ChangingContext.Merged property, given an initial Item that contains a property like:
<property_name type='Some Type' is_null='1'></property_name>, the resulting merged item result is invalid due to the property above having 2 is_null attributes.
I have confirmed that removing the type attribute from the incoming property works as expected.