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

Beginne bei internal/controller/connector_controller.go:111 und verfolge den OAuth2-Codeaustausch, der vom Basic OAuth2-Plugin verwendet wird. Stelle die Verbindung zu einem Shibboleth IDP wieder her und vergleiche die Authentifizierungsmethode des Token-Endpunkts des Clients mit den IDP-Logs. Als abgeschlossen gilt die Aufgabe, wenn das Plugin ohne den gemeldeten 50x-Fehler und den Fehler beim Tokenaustausch eine Verbindung herstellt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.