Set EOF on the final Read of a body with a Content-Length, which
will cause clients to recycle their connection immediately upon
the final Read, rather than waiting for another Read or Close
(neither of which might come).  This happens often when client
code is simply something like:

  err := json.NewDecoder(resp.Body).Decode(&dest)
  ...

Then there's usually no subsequent Read. Even if the client
calls Close (which they should): in Go 1.1, the body was
slurped to EOF, but in Go 1.2, that was then treated as a
Close-before-EOF and the underlying connection was closed.
But that's assuming the user even calls Close. Many don't.
Reading to EOF also causes a connection be reused. Now the EOF
arrives earlier.

This CL only addresses the Content-Length case. A future CL
will address the chunked case.

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/49570044