How do you design a proxy server that supports full HTTP 1.1 support?

It seems that even half of nginx only supports HTTP 1.1 keep-alive requests:

This is an HTTP / 1.0 proxy without the ability to continue requests. (As a result, backend connections are created and destroyed on each request.) Nginx tells HTTP / 1.1 to the browser and HTTP / 1.0 to the backend server. As such, it handles keep-alive in the browser. ( source )

This is you, how to implement it?

The most difficult part, in my opinion, is how to make the answer the same order as in keep-alive mode.