JSONCommunicator: use ZonedDateTime to parse OCPP DateTime (ISO8601)

[JanLapp]

Parser accepted only date time string with time zone 'Z'. Replaced SimpleDateFormat with call to ZonedDateTime#parse so that all complete ISO8601 date time strings will be accepted.

[coveralls]

Coverage decreased (-0.09%) to 56.379% when pulling b2ff35f on JanLapp:json-datetime-fix into e0f87cc on ChargeTimeEU:master.