The Transport's writer to the remote server is wrapped in a
bufio.Writer to suppress many small writes while writing headers and
trailers. However, when writing the request body, the buffering may get
in the way if the request body is arriving slowly.

Because the io.Copy from the Request.Body to the writer is already
buffered, the outer bufio.Writer is unnecessary and prevents small
Request.Body.Reads from going to the server right away. (and the
io.Reader contract does say to return when you've got something,
instead of blocking waiting for more). After the body is finished, the
Transport's bufio.Writer is still used for any trailers following.

A previous attempted fix for this made the chunk writer always flush
if the underlying type was a bufio.Writer, but that is not quite
correct. This CL instead makes it opt-in by using a private sentinel
type (wrapping a *bufio.Writer) to the chunk writer that requests
Flushes after each chunk body (the chunk header & chunk body are still
buffered together into one write).

Fixes #6574

Change-Id: Icefcdf17130c9e285c80b69af295bfd3e72c3a70
Reviewed-on: https://go-review.googlesource.com/10021Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>