Follows the path established in 2d75c8f by porting the rest of
stripe-ruby's tests over to OpenAPI. There are a few other changes here
where I've removed some tests that are duplicated or don't make much
sense, or reorganized how we test certain things, but this commit is
largely the same migration operation applied in bulk a few dozen test
suites.
* Rename the `Update` operation to `Save`
* Add the `update` class method to all saveable resources
* Add tests for update method
* Add tests for plans, invoice items, and application fees
Usage on a top-level collection:
```
Stripe::Customer.list.auto_paging_each do |customer|
puts customer
end
```
Usage on a subcollection:
``` ruby
customer.invoices.auto_paging_each do |invoice|
puts invoice
end
```
We've also renamed `#all` to `#list` to prevent confusion ("all" implies
that all resources are being returned, and in Stripe's paginated API
this was not the case). An alias has been provided for backward API
compatibility.
Fixes#167.
Replaces #211 and #248.
