units: \spacedimensionstrue fails when using unit powers

[Sanjoy Mahajan <sanjoy@mrao.cam.ac.uk>]

I'm considering switching from my units macros to the units module.  In
doing that I'm trying \spaceddimensionstrue (found in the m-units.tex
source), which is the most common convention in physics tables and
books, rather than using \Times to separate quantities with a dot.

But it seems not to take account of the exponent in the first unit when
putting space between the first and second units.  Here's a minimal
example:

\usemodule[units]
\spaceddimensionstrue

\starttext

% this looks fine
\startformula
\Meter\Second
\stopformula

% this needs a thinspace after the superscripted 2
\startformula
\Square\Meter\Second
\stopformula

\stoptext