fix: send client_id on the token request when no auth method is set

[ikawalec]

Fixes #160

Device flow leaves AuthMethod empty (PromptForClientConfig only asks for it on the code/client_credentials/refresh/password/jwt-bearer grants), and AuthenticateClient switches on AuthMethod — so with no auth method the token request carried only grant_type and device_code, and the server answered invalid_client.

Fixing it in AuthenticateClient covers every caller (token, PAR), not just device flow.

Before:

POST /token  body=device_code=DC123&grant_type=urn:ietf:params:oauth:grant-type:device_code

After:

POST /token  body=client_id=my-client&device_code=DC123&grant_type=urn:ietf:params:oauth:grant-type:device_code