Assert that tree exists, fix tests (#358)

* assert that tree exists, fix tests

* make assertions more helpful

index.js

@@ -64,6 +64,8 @@ function choo (opts) {
     const state = _store.state({state: {}})
 
     const tree = _router(state.location.href, state)
+    assert.ok(tree, 'choo.start: the router should always return a valid DOM node')
+    assert.equal(typeof tree, 'object', 'choo.start: the router should always return a valid DOM node')
     _rootNode = tree
     tree.done = done