Issue 6831:	p:columns does not allow to disable sorting for individual columns

git-svn-id: http://primefaces.googlecode.com/svn/primefaces/trunk@11106 5a70e874-a5cb-11dd-8af2-493fb0bb9f7c
possible null reference

git-svn-id: http://primefaces.googlecode.com/svn/primefaces/trunk@11107 5a70e874-a5cb-11dd-8af2-493fb0bb9f7c