@Override
    public void map(ImmutableBytesWritable key, Result value, Context context)
      throws IOException, InterruptedException {
      makeAssertions(key, value);
      context.write(key, key);
    }
Pass the key and value to reduce.
@param key     The key, here "aaa", "aab" etc.
@param value   The value is the same as the key.
@param context The task context.
@throws IOException When reading the rows fails.
hbase-mapreduce/src/test/java/org/apache/hadoop/hbase/mapreduce/MultiTableInputFormatTestBase.java