In particular, this lead to the code accepting invalid ETags as long as
they finished with a '"'.

Also remove a duplicate test case.

Change-Id: Id59db3ebc4e4969562f891faef29111e77ee0e65
Reviewed-on: https://go-review.googlesource.com/39690
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>