RUST-1757 Fix final cursor batch handling

[abr-egn]

RUST-1757

Looks like this broke as part of a refactor a few weeks ago, so the only release it's live in is 2.7.0-beta :)

[isabelatkinson]

good catch! this seems like something we should have test coverage of outside of the benchmarks, would you mind adding a repro of what broke so it doesn't happen again?

[abr-egn]

good catch! this seems like something we should have test coverage of outside of the benchmarks, would you mind adding a repro of what broke so it doesn't happen again?

Added a test :) Was a little harder to poke this behavior than I thought until I realized that advance and the Stream impl don't share logic here - I think this is also why this didn't break any spec tests, as those all use Stream methods. I filed RUST-1761 to merge these.