Other drivers (including the cache) never supported fetch modes that are
not listed in `FetchMode`. Converting between the contants is only done
for PDO, so BC only needs to be restored here. PDO will validate the
fetch mode, we don’t have to.

Fixes #2953.

statements in a rewindable iterator. Skipped the test for iterating
multiple times with a message regarding that iterators are non
rewindable.