Ignore units on FontSize in shapeMarshall

This fixes an issue reported by @aleksandra-tarkowska where text label ROIs break ROI display in web (due to font size units). https://github.com/openmicroscopy/openmicroscopy/pull/3245#issuecomment-65805182

To test, add a text label Shape to an image in Insight, then test that it displays in web.
Both the columns will need to be null (the value and the unit). I'll try to improve this error message.