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 alias:

```ruby
ActiveAdmin.register Aaa::Bbb, as: "FooBar" do
```

As it stands, the resource is never found in the ResourceCollection and you get a lookup error. It appears belongs_to assumes the symbol is an underscored class name. 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?