Fix Plone 4.2 testing regressions with primary fields.

The supermodel marshalling was not triggered automatically in Plone 4.2,
therefore primary fields were not marked properly unless the schema was
grokked.

The fields are now marked manually for Plone < 4.3 in tests.
Merge pull request #27 from 4teamwork/jone-plone-4.2-regressions

Fix Plone 4.2 testing regressions with primary fields.