Improve header checkbox behavior

git-svn-id: http://primefaces.googlecode.com/svn/primefaces/trunk@11862 5a70e874-a5cb-11dd-8af2-493fb0bb9f7c
fixed #7457

git-svn-id: http://primefaces.googlecode.com/svn/primefaces/trunk@11863 5a70e874-a5cb-11dd-8af2-493fb0bb9f7c
fixed #6241

git-svn-id: http://primefaces.googlecode.com/svn/primefaces/trunk@11864 5a70e874-a5cb-11dd-8af2-493fb0bb9f7c