xsrftoken: copy from code.google.com/p/xsrftoken

Change-Id: Idd7bad48e585289740327b00f692fde948a2eeb0
Reviewed-on: https://go-review.googlesource.com/17662
Reviewed-by: David Symonds <dsymonds@golang.org>
Run-TryBot: David Symonds <dsymonds@golang.org>
2 files changed