Problem: mysqld doesn't detect that enum data must be reinserted performing
'ALTER TABLE' in some cases.

Fix: reinsert data altering an enum field if enum values are changed.


mysql-test/r/alter_table.result:
  Fix for bug#23113: Different behavior on altering ENUM fields between 5.0 and 5.1
    - test result.
mysql-test/t/alter_table.test:
  Fix for bug#23113: Different behavior on altering ENUM fields between 5.0 and 5.1
    - test case.
sql/field.cc:
  Fix for bug#23113: Different behavior on altering ENUM fields between 5.0 and 5.1
    - Field_enum::is_equal() introduced, which is called to detect that a field
  is changing by 'ALTER TABLE'.
sql/field.h:
  Fix for bug#23113: Different behavior on altering ENUM fields between 5.0 and 5.1
    - Field_enum::is_equal() introduced, which is called to detect that a field
  is changing by 'ALTER TABLE'.