[FIXED JENKINS-21729] Fix IE8 NPE with respect to sortability

IE8 (and I assume other browsers that use a doctype pre-html5) interpret the span outside of the link, as in HTML4.01 you need the closing tag.
http://www.w3.org/TR/REC-html40/struct/global.html#h-7.5.4

The code was causing a NPE in IE8 and users could not sort the columns.