Basic OAuth2 plugin fails to connect to Shibboleth IDP

I'm trying to connect an Answer instance to my institution's IDP (Shibboleth), but I consistently get a 50x error in browser accompanied by the following entry in the application log:
```
ERROR /go/src/github.com/answerdev/answer/answer_build422575497/vendor/github.com/answerdev/answer/internal/controller/connector_controller.go:111 connector received failed, error info: code exchange failed: oauth2: cannot fetch token: 400
Response: {"error":"invalid_request","error_description":"InvalidEvent"}, response data is:
```

(The "response data is:" part is left intentionally blank, nothing is printed there)

The IDP log shows the following contemporaneous error:
```
Profile Action ValidateClientAuthenticationType: Client '[redacted]' registered client_secret_basic but attempted client_secret_post
A non-proceed event occurred while processing the request: InvalidEvent
Profile Action BuildTokenErrorResponseFromEvent: No mapped event found for InvalidEvent, creating general invalid_request
Profile Action BuildTokenErrorResponseFromEvent: ErrorResponse successfully set as the outbound message
```

Some searching of the IDP configuration confirms that it advertises `token_endpoint_auth_methods_supported` as `[ "client_secret_basic", "client_secret_post", "client_secret_jwt", ... ]` when communicating with OIDC clients, but according to the upstream [mailing lists](http://shibboleth.net/pipermail/users/2019-May/044292.html), the client is expected to identify which supported `token_endpoint_auth_method` it intends to use. I don't believe that the OAuth2 client does that automatically, and there doesn't seem to be a configuration parameter to specify it manually either.

EDIT for version info:
I'm using Answer v1.1.1 pulled from Docker

Commencez par internal/controller/connector_controller.go:111 et suivez l’échange de code OAuth2 utilisé par le plugin Basic OAuth2. Reproduisez la connexion avec un Shibboleth IDP et comparez la méthode d’authentification du point de terminaison de token du client avec les logs de l’IDP. La tâche est terminée lorsque le plugin se connecte sans l’erreur 50x signalée ni l’échec de l’échange de tokens.

Rédigé par le modèle d'indexation à partir du texte de l'issue.