Spacer field with hr=true doesn't show the line in component/template settings

Added a CSS rule to the Isis template which will show the < hr > produced by the spacer field with the option hr="true".
Currently it's invisible due to having a width of 0.

Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30490