fixed NA goodRatings.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/recommenderlab@98 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Improved test.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/recommenderlab@99 edb9625f-4e0d-4859-8d74-9fd3b1da38cb