util/types: convertToMysqlTime will truncate hh:mm:ss part for Date type

[tiancaiamao]

Convert to Date should truncate the hh:mm:ss part.

Fix bug:

mysql> select date("2016-11-11 11:00:00") = date("2016-11-11 12:00:00");
+-----------------------------------------------------------+
| date("2016-11-11 11:00:00") = date("2016-11-11 12:00:00") |
+-----------------------------------------------------------+
|                                                         0 |
+-----------------------------------------------------------+
1 row in set (0.00 sec)

[tiancaiamao]

@andelf @coocood @shenli

[hanfei1991]

LGTM

[coocood]

LGTM