movement.hasCategory('quantity_unit') was always false, it was a
misunderstanding of hasCategory API. This was working anyway because
of the order of edit (which depends on iteration order of **kw arguments
passed to edit), this was only working when quantity unit was set before
resource.

( this was revealed by test_01_quantity_unit_copied from testInvoice
running on python2 with PYTHONHASHSEED=164 )