Use the same timezone (including DST) for both the time and the "offset from UTC" when sending ISO dates

Assume a post whose UTC date is 2010-01-01T00:00:00Z , and a phpBB3 board configured with time zone UTC+1 and DST enabled. The actual ISO date for that post should be 2010-01-01T02:00:00+02:00 , yet the Tapatalk plugin will happily send "2010-01-01T02:00:00+01:00". This is because if formats the time using the correct timezone ($user->timezone + $user->dst) yet then uses a different timezone ($user->data['user_timezone'] WITHOUT $user->data['user_dst']) to add the offset suffix.

This patch makes the plugin use $user->timezone + $user->dst in both cases.

Please verify the client application continues showing the correct dates, though I suspect running phpBB with DST enabled is not a very common configuration.