Use constants from net/http for the status codes

When generating the api.gen.go, we only see one constant from the net/http package:

```
w.WriteHeader(http.StatusNotImplemented)
```

All other places it just uses the raw status code integer value from the spec:

```
w.WriteHeader(504)
```

Would be helpful to get a readable constant name instead of the integer,
especially when you use more than the typical 200 and 404 status codes.
