Fix view_path detection bug of partial

Currently `partial` doesn't work without `view_path` option.

For example,

```
object @result
node(:resources) do |o|
  o.resources.map { |resource| partial("partial_view", object: resource) }
end
```

This code works ver 0.11.0, but now (0.11.4) this raises RuntimeError `Cannot find rabl template 'partial_view' within registered ([]) view paths!`.