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

[Alex-450]

Hey! First of all I wanted to voice some appreciation for this gem, we've been using it for a long time 🙌

We ran into this issue in our app and it was causing postgres to create some inefficient query plans, as the default fraction being set to 0.1 resulted in sequential scans rather than index scans. We ended up patching it like so:

PostgreSQLCursor::Cursor.prepend(
  Module.new do
    def set_cursor_tuple_fraction(frac = 1.0)
      frac = @options.fetch(:fraction, frac)

      @connection.execute("set cursor_tuple_fraction to #{frac}")
    end
  end,
)

I've tested this and it works as expected - fraction is set to 1.0 for these queries. This PR would be a good fix, if there's anything I can do to help releasing let me know :)