Fixed a bug in PropertyColumn introduced by r8780.  PropertyColumn needs access to the underlying class literals in order to get the value to render.

Review at http://gwt-code-reviews.appspot.com/889801

Review by: rjrjr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8794 8db76d5a-ed1c-0410-87a9-c151d255dfc7