Fix issue with error when the context object is not set

I noticed that the hapi-assets plugin fails when the context is not already set in the `reply.view()` call. For example, if you were to do something like:

```js
handler: function(request, reply){
          // Render the view (response.source.context will not be set)
            reply.view('index');
        },
...
handler: function(request, reply){
          // Render the view (response.source.context will be set as we are passing in an object)
            reply.view('index', { title: 'hi!'');
        },
```

This PR addresses that issue.