INTERVALS

ISSUE:
------
Some string functions return one or a combination of the
parameters as their result. Here the resultant string's
charset could be incorrectly set to that of the chosen
parameter.

This results in incorrect behavior when an ascii string is
expected.

SOLUTION:
---------
Since an ascii string is expected, val_str_ascii should
explicitly convert the string.

Part of the fix is a backport of Bug#22340858 for mysql-5.5
and mysql-5.6.