undefined method `require_params' when creating checkout sessions for payments and subscriptions

[mpmenne]

Hey All,

I noticed that I was receiving the following error when I was creating a Stripe::Checkout::Session without using the default params on StripeMock#create_checkout_session

undefined method `require_params'

It looks like a two lines snuck into master without test coverage. Here is one spot and here is another spot where the issue occurs.

This error is fixed by changing require_params to require_param.

I realize that this issue doesn't change the behavior of the library, but it may help the next person who sees this error.

Happy to submit a PR to address if people are open to merging it.