net/http: clarify Request.ContentLength behavior on the client.

net/http: clarify Request.ContentLength behavior on the client.

While you could argue the previous wording technically said that -1 is
an acceptable way to indicate "unknown" on the client, it could be read
as ambiguous. Now it's clear that both 0 and -1 mean unknown.

Change-Id: I3bc5a3fd5afd1999e487296ec121eb548415e6b0
Reviewed-on: https://go-review.googlesource.com/29130
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>