Named cursors on old server versions have a different prefetch behaviour.
This has hidden me the supported range of the 24:00 time format.
Let's have another go at full testing...
`close()` is implicitly called by `__exit__()`, so an exit on error
would run a query on a inerr connection, causing another exception
hiding the original one. The fix is on `close()`, not on `__exit__()`,
because the semantic of the latter is simply to call the former.
Closes#262.
