fix POSKeyError during commit after savepoint rollback

see https://github.com/zopefoundation/ZODB/issues/16
Add test for fix POSKeyError during commit after savepoint rollback

see https://github.com/zopefoundation/ZODB/issues/16
update changelog
Merge pull request #19 from zopefoundation/GH-16

Fix for #16

Thanks!