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.