Datum passed to destroy should be most recent

[curran]

This test fails if added to /test/exit-test.js.

var datum,
    customExit = d3.component("p")
      .destroy(function (selection, d){
        datum = d;
        return selection.transition().duration(10);
      });

tape("Datum passed to destroy should be most recent.", function(test) {
  var div = d3.select(jsdom.jsdom().body).append("div");

  div.call(customExit, "a");
  div.call(customExit, []);
  test.equal(datum, "a");

  div.call(customExit, "a");
  div.call(customExit, "b");
  div.call(customExit, []);
  test.equal(datum, "b"); // Fails here, uses "a"

  test.end();
});