WITH UTF8_UNICODE_CI COLLATION
Problem Description:
When comparing datetime values with strings, the utf8_unicode_ci collation 
prevents correct comparisons. Consider the below set of queries, it is not 
showing any results on a table which has tuples that satisfies the query. 
But for collation utf8_general_ci it shows one tuple.
set names utf8 collate utf8_unicode_ci;;
select * from lang where dt='1979-12-09';

Analysis:
The comparison function is not chosen in case of collation utf8_unicode_ci.
In agg_item_set_converter() because the collation state is having 
"MY_CS_NONASCII" for collation type "utf8_unicode_ci". The conversion 
of the collation is happening for the date field. And because of that 
it is unable to pickup proper compare function(i.e CMP_DATE_WITH_STR).

Actually the bug is accidentally introduced by the WL#3759 in 5.5. 
And in 5.6 it is been fixed by the WL#3664.

Fix:
I have backported the changes from the file strings/ctype-uca.c which 
are related to "utf8" introduced by the WL#3664.
This change helps in choosing the correct comparison function for all 
the collations of utf8 charset.