Add Type_handler::union_element_finalize()

There is a code in sql_union.cc which checks that an Item is OK as a UNION field.

    while ((type= tp++))

    {

      if (type->cmp_type() == STRING_RESULT &&

          type->collation.derivation == DERIVATION_NONE)

      {

        my_error(ER_CANT_AGGREGATE_NCOLLATIONS, MYF(0), "UNION");

        goto err;

      }

    }

Currently, this code has only one purpose: In case of character string data types it makes sure that the aggregated data type has a non-conflicting collation, to disallow this:

mysql> (SELECT 'a' COLLATE utf8_general_ci) UNION (SELECT 'b' COLLATE utf8_unicode_ci);

ERROR 1267 (HY000): Illegal mix of collations (utf8_general_ci,EXPLICIT) and (utf8_unicode_ci,EXPLICIT) for operation 'UNION'

All other built-in data types have no additional requirements on the aggregated data type.
But user defined data types may want to have their own requirements.

This code should go into a new virtual method in Type_handler.