[ResourceBundle] Fixed renderSortingLink not defaulting to property name

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

In the [documentation for Resource Bundle] (http://docs.sylius.org/en/latest/bundles/SyliusResourceBundle/index_resources.html#sorting-your-resources-sylius-resource-sort), it states that the label property is optional for ```sylius_resource_sort```. But there's a bug when using this method with sortable = false, where it will not default to the property name.

I've fixed this bug and added a couple of scenarios to the specs, explaining this behaviour.