Selection not synced properly when using model object

I came across this issue, I would like to share it, may be README could be updated as well, it took me several days to get it working.

Basically, the scenario is that you have an item model with belongsTo category property. you want to edit an item and show categories as a select box, so user can update the category.

you have to use the content in the select value because the belongsTo is a promise:

```
//template.hbs
{{#x-select class="form-control" value=model.Category.content}}

{{#each categories as |category|}}
{{#x-option value=category}}{{category.name}}{{/x-option}}
{{/each}}
{{/x-select}}

```