t-digest: Index out of bounds in MergingDigest

Hi,

The following variant of the testSmallCountQuantile test results in a java.lang.ArrayIndexOutOfBoundsException error. Dropping the last element will have the test pass.

@Test
public void testSmallCountQuantile2() {
    List<Double> data = Lists.newArrayList(3.0, 3.1, 3.0, 3.0,
                                           3.0, 3.0, 3.0, 3.0,
                                           3.0, 3.0, 3.0, 3.0,
                                           3.5, 3.3, 3.4, 3.5,
                                           3.5, 4.0, 2.95, 3.18,
                                           180.0, 3.0, 3.0, 3.0
                                           , 3.0
                                           );
    TDigest td = new MergingDigest(1);
    for (Double datum : data) {
        td.add(datum);
    }
}