Allow resource handlers to completely handle their own responses
Allow resource handlers to completely handle their own responses

Since #64 resource handlers have been exposed to `http.ResponseWriter` objects
and this allows them to return response bodies as necessary. However, when this
happens, `luddite`-level handler wrappers still call `WriteResponse` and this
ultimately leads to output in the standard error log:

```
2017/11/13 08:50:27 http: multiple response.WriteHeader calls
```

This change allows resource handlers to signal that they've handled the complete
response. If they return an HTTP status <= 0 then `luddite` won't otherwise try
to deal with the response.

SpirentOrion/luddite