Add cookie assertion fails on single cookie deletion

[aabdellah]

An HTTP server can instruct remote clients to remove a cookie by setting its value to an empty string. When this happens this assertion fails:

`+ (void)addCookieToSharedCookiesStorage:(NSHTTPCookie *)cookie { [[NSHTTPCookieStorage sharedHTTPCookieStorage] setCookie:cookie];

#if DEBUG NSHTTPCookie *readCookie = [[[NSHTTPCookieStorage sharedHTTPCookieStorage] cookies] lastObject]; NSAssert(readCookie, @"cannot read any cookie after adding one"); #endif }`

It shouldn't attempt to reread a cookie with an empty value.