Add public initializer to HTTPCookieStorage

[agisboye]

The Foundation docs state that HTTPCookieStorage can be subclassed. This, however, is impossible to do with the swift-corelibs-foundation version since there are no publicly accessible initializers.

class MyCookieStorage: HTTPCookieStorage {
    init(someParameter: String) {
        super.init() // Accessible on Darwin but not on other platforms
    }
}

This PR adds an initializer. It has no parameters similar to the Darwin version.
From what I can gather, when a HTTPCookieStorage is initialized, it's completely separate from the shared storage and only lives as long as there is a reference to it (i.e. cookies are not persisted).

[guoye-zhang]

Looks OK to me. -init is inherited from NSObject on Darwin I believe.

@zhenchaoli Does HTTPCookieStorage() also create an in-memory cookie storage on Darwin?

[zhenchaoli]

Seems that on Darwin HTTPCookieStorage() does not have internal backing store, so setting/retrieving cookies are no-ops.

[agisboye]

@guoye-zhang, @zhenchaoli I've changed the behavior such that setting cookies is a no-op (making all the other operations no-ops as well).