allow dst flag to be used for TZInfo::Timezone#ical_timezone, otherwise it could throw TZInfo::AmbiguousTime during a day-light-savings transition.
uses Timezone.default_dst as the default parameter to Timezone#ical_timezone
Merge pull request #43 from zerobearing2/dst-bug

FIXED: throws TZInfo::AmbiguousTime during day-light-savings transition