Issue with preloading, if fetch index is far from visible zone.

[xsahil03x]

Hello! PR #9 fixes the main issue, but there is still an issue with preloading, if fetch index is far from visible zone.

1. Suppose that page key is the id of message [each message has id = text = ordering number, e.g. Message(id=1, text=1)]
2. We load the page with key = 20 (id of first visible message), pageSize = 45, prefetchIndex = 22.
3. It is expected that pager will trigger previous page (with key = 1)
4. We have loaded full page (from 20 to 65), but the prefetchIndex is far from visible zone, we have to scroll to item 20+22 = 42 to trigger prepend. In that case prefetchIndex < itemsCount, meaning that fix with if (prefetchIndex > itemCount) { ... } will be ignored

Originally posted by @dinaraparanid in #8 (comment)