Network Cookie Jar: implement assignment operator.

This change is needed for implementing cookie persistence in Web+ using
the network kit backend.

The current implementation requires the user to unarchive the cookie
jar, then hand it over to the BUrlContext which will copy it to its own
field. This makes the code simpler, but maybe doing a complete copy
(with all the cookies) is an heavy operation and could be avoided.

headers/os/net/NetworkCookieJar.h

@@ -58,10 +58,13 @@ public:
 	virtual	status_t			Unflatten(type_code code,
 									const void* buffer, ssize_t size);
 
+			BNetworkCookieJar&	operator=(const BNetworkCookieJar& other);
+
 	// Iterators
 			Iterator			GetIterator() const;
 			UrlIterator			GetUrlIterator(const BUrl& url) const;
 
+
 private:
 			void				_DoFlatten() const;