added ability for TINY[MEDIUM] text fields 
to be converted to greater subtype during
alter if necessary(altered charset)


mysql-test/r/alter_table.result:
  test result
mysql-test/t/alter_table.test:
  test case
sql/sql_table.cc:
  added ability for TINY[MEDIUM] text fields 
  to be converted to greater subtype during
  alter if necessary(altered charset)