Fix default minutes when using minuteStepping

When using `minuteStepping` without a default date set, the datetimepicker uses a new date object as the base time. The minutes and seconds will thus not align to the minuteStepping units. So if I have minuteStepping set to 15 minutes and it's 3:18 when I create the datetimepicker, I'll be able to choose 3:33, 3:48 but not 3:30 or 3:45.

What this PR does is reset the minutes to a multiple of minuteStepping if it detects minuteStepping is a custom value.