Statistics.of() instance doesn't acknowledge super sets of keys

The default instance of Statistic returned by Statistics.of() doesn't acknowledge keys which contain other keys.

      public boolean isKey(ImmutableBitSet columns) {
        for (ImmutableBitSet key : keys) {
          if (columns.contains(key)) {
            return true;
          }
        }
        return false;
      }

This test should also return true for bit sets which subsume a key, e.g. if A is a key then AB is also a key.