feat: Add extra token request params for authorization code flow

[dominikklein]

In addition to the PR I improved the related test case.

I wrongly thought that the client_id needs also to be added, but this is already done in the rack oauth2 client.

With this e.g. the usage of Auth0 would also work out-of-the-box with this gem.

[dominikklein]

@bufferoverflow Maybe you can check if this is the way to go :-)

[dominikklein]

I wrongly thought that the client_id needs also to be added, but this is already done in the rack oauth2 client.

Ok, I was wrong, with basic client_auth_method it's needed.