Fix for regex pattern deserialization bug

This patch is a workaround for a bug where RegexStringComparator does not apply the DOTALL flag to the regex pattern when deserializing (via Writable#readFields(DataInput in). This issue was fixed (potentially inadvertently) in https://issues.apache.org/jira/browse/HBASE-5667

The bug manifests when filtering a byte array that contains newline or carriage return characters (decimal val 10 and 13 in ISO-8859-1)