Prepared statement column metadata can be wrong with UNION

SELECT 1 UNION SELECT 'abc'

on Prepare, Protocol::send_result_set_metadata() sends type INT to user

on subsequent execution , it sends STRING in metadata.

And a more complicated case

CREATE TABLE t1(i INT);
​SELECT i FROM t1 GROUP BY i WITH ROLLUP UNION ALL SELECT ELT(FOUND_ROWS(), 1) f FROM t1 GROUP BY f WITH ROLLUP;
In this, SELECT the type 3 (MYSQL_TYPE_LONG) changes to type 253 (MYSQL_TYPE_VAR_STRING) from prepare to execute.