Previously, setting raw cookies was not supported without having
to subclass Response and override several methods. This patch
adds official support for this functionality, while also explicitly
disallowing the use of Set-Cookie with certain generic header
manipulation methods that have semantics that are incompatible
with Set-Cookie.

BREAKING CHANGE: Previously, several methods in the Response class
	could be used to attempt to set raw cookie headers. However,
	due to the Set-Cookie header values not being combinable
	as a comma-delimited list, this resulted in an unexpected and/or
	incorrect response being constructed for the user agent in
	the case that more than one cookie was being set.

Fixes falconry#1265