Add Test Coverage for DS.NumberTransform

Transforms are not tested at the `unit` level. Adding tests at the
`unit` granularity allows for regression testing if the
implementation covered by the `acceptance` level tests should change.

packages/ember-data/tests/unit/transform/number_test.js

@@ -0,0 +1,17 @@
+module("unit/transform - DS.NumberTransform");
+
+test("#serialize", function() {
+  var transform = new DS.NumberTransform();
+
+  equal(transform.serialize(null), null);
+  equal(transform.serialize(undefined), null);
+  equal(transform.serialize("1.1"), 1.1);
+});
+
+test("#deserialize", function() {
+  var transform = new DS.NumberTransform();
+
+  equal(transform.deserialize(null), null);
+  equal(transform.deserialize(undefined), null);
+  equal(transform.serialize("1.1"), 1.1);
+});