Properly handle list URLs with query params

Upcoming invoices use a URL of the form
`/v1/invoices/upcoming/lines?customer=cus_4ozMxJvDf1vy39` for their
`url` field. This breaks calls to `Stripe_List` on
`$upcoming_invoice->lines`.
Merge pull request #108 from stripe/lists_with_query_params

Properly handle list URLs with query params