Doesn't return an ActiveResource::Collection

Really useful and easy to use gem. The only issue I'm experiencing is that it makes the resource return an `Array` rather than an `ActiveResource::Collection`. This is especially an issue when using a custom collection parser that extends the functionality of the list of elements.

```ruby
class PostCollection < ActiveResource::Collection
  attr_accessor :next_page
  def initialize(parsed = {})
    @elements = parsed['posts']
    @next_page = parsed['next_page']
  end
end

class Post < ActiveResource::Base
  self.site = "http://example.com"
  self.collection_parser = PostCollection
end
```

Example taken from: http://www.rubydoc.info/github/rails/activeresource/ActiveResource/Collection