Adds stripe key to allowed options in generate_options

Adds :key to slice in generate_options for stripe gateway to allow access token key to be used from stripe connect authorizations to make requests on behalf of user, https://stripe.com/docs/connect/collecting-fees. The rest of the logic existed for this, it just wasn't allowing the key to make it through to the headers call to set the correct auth header.

@odorcicd review please :)