Bug #11936829: DIFF. BETWEEN MYSQL.USER (AUTHENTICATION_STRING) IN 
  FRESH AND UPGRADED 5.5.11
             
There was no modification for pre 5.5.11 builds that had 
authentication_string.  Thus the column was not upgraded 
by mysql_upgrade.
                         
Fixed by adding an ALTER TABLE MODIFY to update an existing 
column to the latest type version.
                                     
Test suite added.