Release 6.20.1: Changed TimeZone of Conversion Interface

[changerequest]

The TimeZone in the result (json) always contains the given TimeZone of the iCal. That makes no sense if the times are calculated with another TimeZone.

The returning TimeZone will aways represent the correct TimeZone which is used for the calculation. This may be the original TimeZone of the iCal file or not.