WritableComparator newKey() fails for NullWritable

It is not possible to use NullWritable as a key in order to suppress key value in output.

Syndrome exception:
Caused by: java.lang.IllegalAccessException: Class org.apache.hadoop.io.WritableComparator can not access a member of class org.apache.hadoop.io.NullWritable with modifiers "private"

The problem is that NullWritable is a singleton and does not provide public non-parametric constructor. The following code in WritableComparator causes the exception: return (WritableComparable)keyClass.newInstance();

Proposed simple solution is to use ReflectionUtils instead (it requires modification as well).

This issue is probably related to HADOOP-2922