Do not use AccessToken parameter to specify key for connect

`AccessToken` is currently accepted as a parameter for certain types of requests where we expect you to use a Connect token (e.g. creating a token from a shared customer, adding an application fee to something).  I think putting this in the bindings is confusing since it implies that the `AccessToken` parameter is like other parameters (i.e. passed in the POST body) rather than simply being another name for the API key.  The validation this provides is largely unnecessary -- the API itself is very capable of handling this logic and returning a sane error.  I believe Connect users should instead be creating a separate `client.Api` instance with the desired key.  

Note that this is in fact consistent with our other bindings.  The `ACCESS_TOKEN` positional argument we show [e.g. for shared customers](https://stripe.com/docs/connect/shared-customers#making-a-charge) in fact [corresponds to an `api_key` parameter](https://github.com/stripe/stripe-ruby/blob/master/lib/stripe/api_operations/create.rb#L5), not something special for connect.  I think the inclusion of this parameter in our other bindings is a reflection of a bad design decision there to not allow creating separate client instances.

r? @cosn