Fixed error when getting instance_url from resources without id

[hirokiky]

Creating resource without id and getting instance_url will raise TypeError

Traceback (most recent call last):
  File "/Users/user1/dev/payjp-python/payjp/test/test_resources.py", line 201, in test_instance_url_without_id
    resource.instance_url()
  File "/Users/user1/dev/payjp-python/payjp/resource.py", line 259, in instance_url
    raise error.InvalidRequestError
TypeError: __init__() takes at least 3 arguments (1 given)

I added a test to detect it and fixed.

Cases

I think this case is not rare.
Because it will happen on calling retrieve with None.

subscription = ...  # Creating local object
subscription.payjp_id = None  # and it has `id` on payjp as optional (like Null-able field on Django model)

import payjp
payjp.Subscription.retrieve(subscription.payjp_id)  # raise TypeError

[wozozo]

🙇‍♀️