browser: Add user-agent header filter to gorilla mux route (#8040)

When a peer client which higher version sends a request to a peer
server with lower version, the returned status code is 200 OK instead
of 405 code. The reason is that the peer client request reaches the
browser handler, which registers itself by '/minio' route but without
any other constraints. Adding filtering by user agent header to the
browser route so internal requests to old endpoints versions return
405 error code.

cmd/test-utils_test.go

@@ -1232,6 +1232,7 @@ func newWebRPCRequest(methodRPC, authorization string, body io.ReadSeeker) (*htt
 	if err != nil {
 		return nil, err
 	}
+	req.Header.Set("User-Agent", "Mozilla")
 	req.Header.Set("Content-Type", "application/json")
 	if authorization != "" {
 		req.Header.Set("Authorization", "Bearer "+authorization)