net/http/httputil: make ReverseProxy forward 1xx responses

[bradfitz]

[Continuation of #17739]

The ReverseProxy does not forward 1xx responses currently.

https://tools.ietf.org/html/rfc7231#section-6.2

A proxy MUST forward 1xx responses unless the proxy itself requested the generation of the 1xx response.

[bradfitz]

This requires #26089 (Server/Handler/ResponseWriter doesn't allowing sending arbitrary 1xx responses).

[gopherbot]

Change https://golang.org/cl/269997 mentions this issue: net/http: allow sending 1xx responses

[gopherbot]

Change https://golang.org/cl/270157 mentions this issue: http2: allow sending 1xx responses

[gopherbot]

Change https://golang.org/cl/291029 mentions this issue: http2: allow sending 1xx responses