Fix `Dispatcher.Register` example tests (#1916)
Fix `Dispatcher.Register` example tests (#1916)

Turns out that example tests won't run unless there's the following
output comment.

```
// Output:
```

This includes tests that have no output. This change runs and fixes
the broken examples.

- https://blog.golang.org/examples

Fixes #1911
yarpc/yarpc-go,yarpc/yarpc-go