fix: set client CAs for mTLS auth (#437)

The new TLS config introduced in PR #414 does not set client CAs.
So when enabling mTLS authentication, client requests fail with
`certificate signed by unknown authority`.
This commit fixes this by using the root CAs also for
authenticating mTLS clients.

Ref: #414

kesconf/file.go

@@ -150,6 +150,7 @@ func (f *File) TLSConfig() (*tls.Config, error) {
 		Certificates: []tls.Certificate{certificate},
 		NextProtos:   []string{"h2", "http/1.1"},
 		RootCAs:      rootCAs,
+		ClientCAs:    rootCAs,
 	}, nil
 }