net/http: accept empty method in Transport again

net/http: accept empty method in Transport again

Fix regression from https://golang.org/cl/16829 ("require valid methods
in NewRequest and Transport.RoundTrip").

An empty string is a valid method (it means "GET", per the docs).

Fixes #13311

Change-Id: I26b71dc4ccc146498b5d7e38fbe31ed11dd5a6cf
Reviewed-on: https://go-review.googlesource.com/16952
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>