net/http: add NoBody, don't return nil from NewRequest on zero bodies

net/http: add NoBody, don't return nil from NewRequest on zero bodies

This is an alternate solution to https://golang.org/cl/31445

Instead of making NewRequest return a request with Request.Body == nil
to signal a zero byte body, add a well-known variable that means
explicitly zero.

Too many tests inside Google (and presumably the outside world)
broke.

Change-Id: I78f6ecca8e8aa1e12179c234ccfb6bcf0ee29ba8
Reviewed-on: https://go-review.googlesource.com/31726
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>