The correct way to get Jetty to send HTTP / 1.0 responses is to send HTTP / 1.0 requests to the client.
As a rationale, consider this paragraph from the HTTP 1.1 specification:
, , HTTP- "HTTP/1.1", , , . , , HTTP- "HTTP/1.1" , HTTP/1.0. , HTTP-, . RFC 2145.
, HTTP/1.1, HTTP/1.1. , HTTP... .