Fixed datetime-local to support DST.

Made changes to fix issues discussed in https://github.com/aldeed/meteor-autoform/issues/344. This removes the need to pass an offset if we're working with local time, but I suspect we should discuss the implications and whether these changes "fix" the issue for me but break the expected behaviour for everyone else. From my understand, `moment()` alone will handle all local time loading and saving without the need to call `zone()` or append a custom offset - both of which can cause issues for DST in the same timezone.