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the ratio was 100%. We want to separate it in two different cases:
 - ratio is 0 -> the line should not be created.
 - ratio is not set (ie. None) -> this means that no ratio apply here, it's a
   simple amount, so ratio default to 100%
Move this in the default calculation script so that it's possible to
change the behaviour.
Add tests for those two cases (price = None and price = 0)



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@21632 20353a03-c40f-0410-a6d1-a30d3c3de9de