Serialize ResourceResponses using WebKit types
https://bugs.webkit.org/show_bug.cgi?id=136545

Fix the failing webarchive tests.

* platform/network/mac/ResourceResponseMac.mm:
(WebCore::ResourceResponse::initNSURLResponse): Map empty text encoding name to nil NSString.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@173360 268f45cc-cd09-0410-ab3c-d52691b4dbfc