Excluded ASCII from ENC_KNOWN,  as Defn.h does too.  The warning in bmerge() is updated with TO DOs to revisit, and only now occurs for truly known encodings (latin1 or UTF8).

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1153 edb9625f-4e0d-4859-8d74-9fd3b1da38cb