Create resources based on the element if no href links to the object

cc/ @bhelx 

When previewing a subscription change, the invoice is passed back in the body of the response with an `href` of `""`. If this is the case, we can materialize the `Invoice` from those attributes instead of following a nonexistent link.

tests:

```python
sub = recurly.Subscription.get('<uuid>')
sub.quantity = <new quantity>
sub.preview()
for li in sub.invoice.line_items:
  print "line item: %s" % li
```