ResourceCollection lookup should also search against resource_name

Hi. I hit a problem with ```belongs_to :foo_bar``` where ```:foo_bar``` refers to a resource name:

```ruby
ActiveAdmin.register Foo::Bar, as: "FooBar" do
```

As it stands, the resource is never found in the ResourceCollection and you get a lookup error. It appears belongs_to doesn't seem to be capable of handling a resource aliases. I've patched the find predicate to check the resource_name as well as the resource_class when searching the collection. 

What do you reckon? Is this sensible?