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

[cbarton]

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:

sub = recurly.Subscription.get('<uuid>')
sub.quantity = <new quantity>
sub.preview()
for li in sub.invoice.line_items:
  print "line item: %s" % li

[bhelx]

👍