Kneed beams when changing stem direction \once

[Daniel Rosen]

> I'm not top posting.

%{ In the example below, the two pairs of beamed eighth notes should appear
identical due to the \override command; however, it appears that the \once
command preceding the \stemDown for the first pair is preventing that. %}

\version "2.16.1"
{
  \override Beam #'auto-knee-gap = #30
  \once \stemDown
  f''8[ d']
  \stemDown
  f''8[ d']
}