Accordion: fixes #9264

Using .show method and outerHeight property to force height calculation when
heightStyle='auto'
Using `.outerHeight()` here makes the panels too large since we're setting the value via `.height()`. The two methods have to match; since both methods account for `box-sizing`, there shouldn't be a need to change.

You can see the problem in the default demo; there's extra spacing below the content of panel 3 (the tallest content). Probably the easiest way to see the difference is to just visually compare it with http://view.jqueryui.com/master/demos/accordion/default.html.