Adding attributes to wrapped element in XML collection

We currently need to be able to add attributes to the element wrapping collections in XML, and from the code and the documentation I couldn't find an elegant way of doing this.

This will allow you to do the following:

```ruby
collection :songs, :as => :song, :wrap => :songs, :wrap_attributes => {:foo => :bar}
```

which would generate the XML:

```xml
<?xml version="1.0"?>
<songs foo="bar">
  <song>
    <name>Thinning the Herd</name>
  </song>
</songs>
```