Fix using auto_paging_iter() with expand: [...]

[xavdid-stripe]

Why

When you called auto_paging_iter() on a list that you fetched with expand, the server would give a 400 error. Ultimately, this was caused by the SDK making a malformed request with similar-but-different expand values.

When you turn a response into a StripeObject, we store the request url that created that response. That url includes any query params. So, given this code:

lines = stripe.Invoice.upcoming_lines(
    limit=3, 
    customer='cus_ROQYYBI1BraV53',
    expand=['data.price.product']
)

hits the url: /v1/invoices/upcoming/lines?customer=cus_ROQYYBI1BraV53&expand%5B%5D=data.price.product

In addition to the URL, Python's StripeObject also stores the params used to create the object. On subsequent requests with the same object (e.g. when paging through a list) the SDK adds the original params to paging related ones (such as starting_after) to the original url to make the request.

Because the server returned the params and Python still has the params, we end up with duplicates in the url:

https://api.stripe.com/v1/invoices/upcoming/lines?customer=cus_ROQYYBI1BraV53&expand%5B%5D=data.price.product&limit=3&customer=cus_ROQYYBI1BraV53&expand[0]=data.price.product&starting_after=il_tmp_1QVdh7JoUivz182DmwLpUU1y

Not only is customer duplicated, there's both expand[] and expand[0] keys. The latter is what's causing the issue. Clients can send expand[0]=a&expand[1]=b or expand[]=a&expand[]=b, but can't mix.

The server sends extend[] in its url response and python adds indexes, so the clobbering was the root of the issue.

To fix, I parsed out the existing query params from a url and merged them with params, which avoids duplicates. I also merged expand values just in case someone is expanding something new on the next_page() call that wasn't expanded originally.

This approach is a little kludgy, but does work and fix the bug. I'm open to other suggestions though!

What

• when making a GET or DELETE request with additional params, deduplicate params.
• add test

Testing

import stripe
stripe.api_key = "sk_test_..."
# set up customer with more than one page of invoice items
customer = stripe.Customer.create()
for i in range(5):
    stripe.InvoiceItem.create(
        customer=customer.id,
        price="price_1PVu24JoUivz182D067aXjgy"
    )
    print(f"Created invoice item {i}")
print(f"Done creating invoice items for {customer.id}, now listing them")
# try to iterate over all invoice items
lines = stripe.Invoice.upcoming_lines(
    limit=3, 
    customer=customer.id,
    expand=['data.price.product']
)
for line in lines.auto_paging_iter(): # errors after 3
   print(f"{line.id}")

---

fixes RUN_DEVSDK-1427

[xavdid-stripe]

this section is mostly removing % formatting, but I also removed the duplicate foo=bar since that's not really a supported pattern.