Add POST support to XHR as well as .status and statusText support

sky/tests/framework/xmlhttprequest/xhr-does-not-exist.sky

+<sky>
+<import src="/sky/tests/resources/chai.sky" />
+<import src="/sky/tests/resources/mocha.sky" />
+<import src="/sky/framework/xmlhttprequest.sky" as="XMLHttpRequest" />
+<script>
+describe("xmlhttprequest", function() {
+  it("should call onerror when endpoint does not exist", function(done) {
+    var xhr = new XMLHttpRequest();
+    xhr.open("GET", "does_not_exist.html");
+    xhr.onerror = function() {
+      assert.fail("onload", "onerror", "onerror should not be called.");
+      done();
+    }
+    xhr.onload = function() {
+      // Missing files are application-level errors, not network errors
+      // so onload fires, not onerror.
+      assert.equal(xhr.status, 404);
+      assert.equal(xhr.statusText, "HTTP/1.1 404 Not Found",
+          "status text should also be 404");
+      done();
+    }
+    xhr.send();
+  });
+});
+</script>
+</sky>