Fix isUTC and UTC timezone

[mmso]

Turns out that

BEGIN:VEVENT
DTSTART;TZID=Etc/UTC:20190101T000000
DTEND;TZID=UTC:20190101T020000
END:VEVENT

Would get parsed as:

{
            component: 'vevent',
            dtstart: {
                value: { year: 2019, month: 1, day: 1, hours: 0, minutes: 0, seconds: 0, isUTC: false },
                parameters: { tzid: 'Etc/UTC' }
            },
            dtend: {
                value: { year: 2019, month: 1, day: 1, hours: 2, minutes: 0, seconds: 0, isUTC: true },
                parameters: { tzid: 'UTC' }
            }
        }

Which would make some of the places were we're checking !tzid unneccessarily convert UTC to UTC. Not sure if this PR is worth it.