Issue warning on joins with character columns and different encodings (bug #5266, #5159)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1096 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Forgot to uncomment the part issuing the warning… (related to last commit).

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1097 edb9625f-4e0d-4859-8d74-9fd3b1da38cb