Make it possible to change the default timezone

[w3stling]

Make it possible to change the default timezone to use if no timezone information exist in the timestamp string. Default UTC is used.

Example pubDate string 2023-08-11T07:17:52 is default treated as a UTC timestamp.

For changing the default timezone to Australia/Sydney:

var items = new RssReader().setDateTimeParser(new DateTime("Australia/Sydney"))
                           .read(URL)
                           .collect(Collectors.toList());