fix: requests without bucket should route to the original router (#9541)

requests in federated setups for STS type calls which are
performed at '/' resource should be routed by the muxer,
the assumption is simply such that requests without a bucket
in a federated setup cannot be proxied, so serve them at
current server.

cmd/generic-handlers.go

@@ -679,9 +679,12 @@ func (f bucketForwardingHandler) ServeHTTP(w http.ResponseWriter, r *http.Reques
 
 	bucket, object := request2BucketObjectName(r)
 
-	// ListBucket requests should be handled at current endpoint as
-	// all buckets data can be fetched from here.
-	if r.Method == http.MethodGet && bucket == "" && object == "" {
+	// Requests in federated setups for STS type calls which are
+	// performed at '/' resource should be routed by the muxer,
+	// the assumption is simply such that requests without a bucket
+	// in a federated setup cannot be proxied, so serve them at
+	// current server.
+	if bucket == "" {
 		f.handler.ServeHTTP(w, r)
 		return
 	}