Here is a quote from the HTTP RFC:
- 4xx: Client Error - The request contains bad syntax or cannot
- 5xx: Server Error - The server failed to fulfill an apparently
Since it's a client error to send the wrong header in the HTTP request I'd expect the server to recognize the client's mistake and send some 4xx error code.
Thanks for the prompt response (and apologies for taking time to get back to you, was on short vacation).