Add User-Agent header to OAuth requests

[whyscream]

Jonas at Mollie requested to add User-Agent headers to all API requests. We are already doing that for all Mollie data requests, but during the OAuth setup we use a different client (OAuth2Session from requests-oauthlib), which doesn't use the custom User-Agent when refreshing tokens etc.

NB We also have the X-Mollie-Client-Info header, we can add that too.