net/http: add corner cases for readCookiesTests

net/http: add corner cases for readCookiesTests

The following corner cases for readCookiesTests are tested now:
 - An extra cookie delimiter ";" at the end of a Cookie header
 - An empty Cookie header

Change-Id: Id8028b448e4182613fb261bf0903efc98cbf4997
Reviewed-on: https://go-review.googlesource.com/c/go/+/164702
Reviewed-by: Volker Dobler <dr.volker.dobler@gmail.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>