Fixed handling of time zones for Routing Requests in REST API (#1373)

Fixed so that if only 'time' parameter is specified (not 'date'), then default timezone used
is still that of the graph being served (unless user specifically requests
another timezone in the 'time' parameter).

Previously, in this case of using the time parameter and no date, unless the user specifically appended a timezone (e.g. +04:00) to time parameter, time in request was interpreted to be in that of the server running OTP, not the graph.

See issue #1373 .