net/http: conservatively flush Transport request headers by default

This changes the http.Transport to flush the bufio.Writer between
writing the request headers and the body.

That wasn't done in the past to minimize the number of TCP packets on
the wire, but that's just an optimization, and it causes problems when
servers are waiting for the headers and the client is blocked on
something before reading the body.

Instead, only do the don't-flush optimization if we know we're not
going to block, whitelisting a set of common in-memory Request.Body
types. (the same set of types special-cased by http.NewRequest)

Fixes #22088

Change-Id: I7717750aa6df32dd3eb92d181b45bc7af24b1144
Reviewed-on: https://go-review.googlesource.com/114316


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>