MAHOUT-1285: Arff loader can misparse string data as double

git-svn-id: https://svn.apache.org/repos/asf/mahout/trunk@1546624 13f79535-47bb-0310-9956-ffa450edef68
MAHOUT-1285: Return NaN if a string

git-svn-id: https://svn.apache.org/repos/asf/mahout/trunk@1546636 13f79535-47bb-0310-9956-ffa450edef68
MAHOUT-1285: Adding entry to CHANGELOG

git-svn-id: https://svn.apache.org/repos/asf/mahout/trunk@1546637 13f79535-47bb-0310-9956-ffa450edef68