This is just an issue with your test case actually.
What this is saying is that the HTTP PUT request you made in your test case got a 302 response. The 302 response is the redirection response code. If you look at your update controller function, this is exactly what you tell it to do:
When you hit the update function from your browser, it gets the 302 status code and immediately redirects to the show page, which can make it less obvious that two independent HTTP requests happen. However in your test case there isn’t an automatic redirect follow that happens.
