Fixed handling of time specification for Routing Requests in REST API 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
antother timezone in the 'time' parameter).

See issue #1373 .
Merge pull request #1446 from PatSunter/fix_timezone_handling_rest_api

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