Remove early return from `set_cursor_tuple_fraction`

[xathien]

The early return removed the ability to override Postgres' default fraction of 0.1 with 1.0. A workaround was to use 0.99999 but that's a little bit ugly, so let's just always set the tuple fraction.

Alternatively, we could check the current fraction on the @connection and only change it if it's different, which would also allow us to revert the temporary fraction change afterward, but that's an adjustment for another PR.

Partially fixes #49