There are several github issues that are both useful and relatively easy to implement. Those are marked with "easy" label ( https://github.com/pgjdbc/pgjdbc/issues ).
Does anybody want to give it a try?
The good part is the next release is coming soon (~ a couple of weeks), so it won't take long for your fix to appear in the official release.
Just in case you did not know: as you file a pull request, the system automatically runs the tests across various PostgreSQL and Java versions, so you can implement a fix even without having a database instance.