`default` property for `m.request`, with tests

Wrote passing tests to assert that
1. [Returned `m.prop` is immediately populated with the default value](https://github.com/barneycarroll/mithril.js/blob/71a6321c467ae33cfaeff104982418b53ca6adf3/tests/mithril-tests.js#L1690-L1693)
2. [Returned `m.prop` receives is populated with new values when XHR resolves](https://github.com/barneycarroll/mithril.js/blob/71a6321c467ae33cfaeff104982418b53ca6adf3/tests/mithril-tests.js#L1694-L1698)

…But running both or either of these tests breaks the [`m.startComputation`/`m.endComputation` test](https://github.com/barneycarroll/mithril.js/blob/71a6321c467ae33cfaeff104982418b53ca6adf3/tests/mithril-tests.js#L1995-L2012), which complains of

```bash
TypeError: Cannot read property 'nodeValue' of undefined