net/http: Don't set Content-Length: -1 when responding to a POST

net/http: Don't set Content-Length: -1 when responding to a POST

Fixes an issue where Response.Write writes out a Content-Length: -1
header when the corresponding Request is a POST or PUT and the
ContentLength was not previously set.

This was encountered when using httputil.DumpResponse
to write out the response from a server that responded to a PUT
request with no Content-Length header. The dumped output is
thus invalid.

Change-Id: I52c6ae8ef3443f1f9de92aeee9f9581dabb05991
Reviewed-on: https://go-review.googlesource.com/9496
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>