union.result, union.test:
  Adding test case.
item.cc:
  Allow safe character set conversion in UNION
  - string constant to column's charset
  - to unicode
  Thus, UNION now works the same with CONCAT (and other string functions)
  in respect of aggregating arguments with different character sets.


sql/item.cc:
  Allow character set conversion in UNION
  - string to column's charset
  - to unicode
  Bug#15949 union + illegal mix of collations (IMPLICIT + COERCIBLE)
mysql-test/t/union.test:
  Adding test case.
mysql-test/r/union.result:
  Adding test case.