FauxResponse: Honor cookie options, and other cleanup

FauxResponse: Honor cookie options, and other cleanup

We have this nice class for unit testing cookie-setting, but the
cookie-setting method ignores all the parameters! Fix that. Also provide
accessors to check the entire set of cookie data, and the set cookies as
a whole.

While this does change the semantics of FauxRequest::getcookie() in that
the name now needs to be prefixed, no extension in Gerrit uses this
method so we should be fine.

Also clean up the case of the setCookie and getCookie methods while
we're at it. Since PHP method names are case-insensitive, this doesn't
even break compatibility with extensions.

Change-Id: Ib44a074bf9796bc0b470d557e39465792f399d30