Support on HTTP Basic Auth protected sites

[sbor23]

Use case:
I want to protect a site from public access. For this I add HTTP Basic Auth to the web server.

Expected:
Entering the site, supplying the HTTP basic auth credentials and the page loads.
Starting the Authorization Code Flow works too.

Actual:
Login with Authorization Code Flow fails with Keycloak, because the Authorization: Basic ... is supplied.
This confused Keycloak, which returns 400 with error=invalid_client_credentials.