Custom select menu header corner styling

[jaspermdegroot]

The header (placeholder) of a custom select menu also inherits the border-radii of the popup, while this is not desired. This is a regression from e87386d .
The issue is visible when you open the second and third example in the section "Placeholder options" at this page: http://jquerymobile.com/test/docs/forms/selects/custom.html

I will commit a fix for this, so this ticket is just to have an issue to refer to.