Until recently, we always permitted an empty string to NewRequest.
Keep that property, since it broke tests within in Google when trying
out Go 1.6, and probably would've broken others too.

Change-Id: Idddab1ae7b9423d5caac00af2c897fe1065b600b
Reviewed-on: https://go-review.googlesource.com/17699Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>