Makes TextBoxBase follow the set(null)-to-clear convention.  Punches
up CheckBox docs to make it clear it doesn't do so, and why, and adds
test to keep it that way.

reviewer: ajr
submitter: rjrjr



git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.6@4263 8db76d5a-ed1c-0410-87a9-c151d255dfc7