Sometimes special 0 ENUM values was ALTERed to normal
empty string ENUM values.

Special 0 ENUM value has the same string representation
as normal ENUM value defined as '' (empty string).
The do_field_string function was used to convert
ENUM data at an ALTER TABLE request, but this
function doesn't care about numerical "indices" of
ENUM values, i.e. do_field_string doesn't distinguish
a special 0 value from an empty string value.

A new copy function called do_field_enum has been added to
copy special 0 ENUM values without conversion to an empty
string.


sql/field_conv.cc:
  Fixed bug #29251.
  The Copy_field::get_copy_func method has been modified to
  return a pointer to the do_field_enum function if a conversion
  between two columns of incompatible enum types is required.
  The do_field_enum function has been added for the correct
  conversion of special 0 enum values.
mysql-test/t/type_enum.test:
  Updated test case for bug #29251.
mysql-test/r/type_enum.result:
  Updated test case for bug #29251.