Replace generic ColumnModifier enum with more specific SortOrder enum.  Ensure that we always pass through either SortOrder.ASC or SortOrder.DESC but never null.
https://issues.apache.org/jira/browse/PHOENIX-32
Reviewed by jamestaylor.
Fix NPE introduced by my previous fix for Phoenix-32 - SortOrder was being passed in as null instead of ASC.
https://issues.apache.org/jira/browse/PHOENIX-32