Fixes Issue 799.
Changes TextBoxBase to normalize the behavior of setText across browsers.
Calling it with null will always result in an empty string being used as
the text value. Also, a test case was added for ongoing verification.

Patch by: bobv
Review by: knorton



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@1058 8db76d5a-ed1c-0410-87a9-c151d255dfc7