There are two kinds of API operations: collection and element specific.
The signature between the two is slightly different:
- **collection**: (params, opts)
- **element specific**: (id, params, opts)
If a user doesn't realize the difference, they may attempt to use the
collection signature when performing an element specific operation like:
```
Stripe::PaymentIntent.cancel('pi_1234', 'sk_test_key')
# Results in an error: NoMethodError: undefined method `key?' for "sk_test"
```
The resulting error message isn't very useful for debugging.
Instead,this PR adds a message letting the user know what it's expecting:
`request params should be either a Hash or nil (was a String)`
299e9ea0ab