return sql.NullTime if it available

[shogo82148]

Description

related to #960 (comment)

Will rows.ColumnTypes() (first value from db.Query()) return sql.NullTime instead of mysql.NullTime too?

My answer is yes.
rows.ColumnTypes() should return sql.NullTime.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[shogo82148]

Can we drop this line?

mysql/driver_test.go

Line 2849 in 73dc904

dsn + "&parseTime=false",

I think it is misuse of mysql.NullTime or sql.NullTime.
They should be treated like time.Time, and used with the parseTime=true option.

previous discussion is here #1049 (comment)

[shogo82148]

Thanks for reviewing, but now tests fail.
I will fix it.