behavior when extending fields that were fully part of a
multi-part key.


mysql-test/r/key.result:
  Update results
mysql-test/t/key.test:
  Add test for behavior of extending fields in a multi-part key
  that were defined with a partial length the same as their
  field length.
sql/sql_table.cc:
  Reset key_part_length when old field length was the same as the
  old key_part_length.