extraNatural and key changes

[David Bobroff]

When using 'extraNatural = ##f' key cancellations at key changes are not 
complete.  Key signature elements which change between sharp and flat on 
the same note names are not canceled.

Feature?
Known issue?
Bug?

-David

\version "2.13.0"

\score {
  \relative {
    \set Staff.extraNatural = ##f
    \key as \major
    s1
    \key e \major
    s1
    \key des \major
  }
}