assignment to single keys in staff-staff-spacing

[Keith OHara]

Dear Bug List,
  It looks like the ability to assign to single keys broke sometime since 
2.13.39
  The single-key assignment works for other properties, like 
nonstaff-nonstaff-spacing; only staff-staff-spacing has the problem.
-Keith

\version "2.13.42"
{ <<
  \new Staff \with {
    \override VerticalAxisGroup #'staff-staff-spacing
      #'basic-distance = #12  % produces warning below, has no effect
%     = #'( (basic-distance . 12) ) % spreads staves as expected
  } c''
  \new Staff c'
  %{%} >> }

%{ warning: type check for `staff-staff-spacing' failed; value `((basic-distance . 
12) . #<primitive-procedure 
ly:axis-group-interface::calc-staff-staff-spacing>)' must be of type `list'
%}