net/http: call TCPConn.SetKeepAlive on Transport

[bradfitz]

We enable TCP keep-alives by default for the http Server, but not the HTTP Client.

Seems like we should also enable it by default for the HTTP/1 and HTTP/2 clients.

(or perhaps just HTTP/1, and HTTP/2 can use PING frames with a timeout)

/cc @tombergan

[jasdel]

Would enabling TCP keep-alive by default on the HTTP Client be equivalent to using a http.Transport's dialer with net.Dialer.KeepAlive > 0 behavior of using SetKeepAlive and SetKeepAlivePeriod to set the keep-alive? Or would only SetKeepAlive be used by the updated HTTP Client's default behavior?

With this default behavior change, would omitting net.Dialer.KeepAlive or setting its value to 0, disable TCP keep-alive like the behavior the current net.Dialer?

[bradfitz]

Nevermind, I actually already did this in 2014 in 49beb23 for #3362