Floating Times are not handled correctly on expand-requests 

A floating date/date-time gets incorrectly expanded to a UTC-version of the date/date-time.

According to the [RFC](https://tools.ietf.org/html/rfc4791#section-7.3) these date/date-times should get processed using one of the following timezones (in that order if available):
* CALDAV:timezone XML element in the request body
* CALDAV:calendar-timezone property
* the servers choice (kind of what is happening now)

**.ics**
```
BEGIN:VCALENDAR
VERSION:2.0
CALSCALE:GREGORIAN
BEGIN:VEVENT
CREATED:20140701T143658Z
UID:dba46fe8-1631-4d98-a575-97963c364dfe
DTEND:20141108T073000
TRANSP:OPAQUE
SUMMARY:Floating Time event, starting 05:30am Europe/Berlin
DTSTART:20141108T053000
DTSTAMP:20140701T143706Z
SEQUENCE:1
END:VEVENT
END:VCALENDAR
```

**calendar-timezone property**
```
BEGIN:VCALENDAR
VERSION:2.0
CALSCALE:GREGORIAN
BEGIN:VTIMEZONE
TZID:Europe/Berlin
BEGIN:DAYLIGHT
TZOFFSETFROM:+0100
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
DTSTART:19810329T020000
TZNAME:GMT+2
TZOFFSETTO:+0200
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0200
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
DTSTART:19961027T030000
TZNAME:GMT+1
TZOFFSETTO:+0100
END:STANDARD
END:VTIMEZONE
END:VCALENDAR
```

**expected result for an expand-request**
```
BEGIN:VCALENDAR
VERSION:2.0
CALSCALE:GREGORIAN
BEGIN:VEVENT
CREATED:20140701T143658Z
UID:dba46fe8-1631-4d98-a575-97963c364dfe
DTEND:20141108T063000Z
TRANSP:OPAQUE
SUMMARY:Floating Time event, starting 05:30am Europe/Berlin
DTSTART:20141108T043000Z
DTSTAMP:20140701T143706Z
SEQUENCE:1
END:VEVENT
END:VCALENDAR
```

**actual result**
```
BEGIN:VCALENDAR
VERSION:2.0
CALSCALE:GREGORIAN
BEGIN:VEVENT
CREATED:20140701T143658Z
UID:dba46fe8-1631-4d98-a575-97963c364dfe
DTEND:20141108T073000Z
TRANSP:OPAQUE
SUMMARY:Floating Time event, starting 05:30am Europe/Berlin
DTSTART:20141108T053000Z
DTSTAMP:20140701T143706Z
SEQUENCE:1
END:VEVENT
END:VCALENDAR
```

See also added unit test.