Accordion fillSpace not accounting for header margins

This is similar to #4267, but just one specific problem with (in my use cases, at least) a very trivial solution.

If the accordion headers have a non-zero margin, fillSpace consistently calculates the incorrect height for the content div.

In r2307, changing line 182 from:

maxHeight -= $(this).outerHeight();

maxHeight -= $(this).outerHeight(true);