In getSizeFromImageDisplay, accessors must be used without default 0 parameter
because it would override the default value from property sheet,
and the user may not have overriden these values in his preferences.

Add a unit test to test that behaviour of preference accessor.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@29505 20353a03-c40f-0410-a6d1-a30d3c3de9de