Fix parsing issue with IPv6 format in Poco::Net::IPAddress

After successful parsing of IPv6 formatted string into IPv6AddressImpl object, during creation of IPAddress object out of it the "scope" (part coming after "%" sign in IPv6 format) is missed out, which brings to point that  Link-local IPv6 address does not contain zone indices hence it's invalid.

Problem encountered and fixed under Mac OS X platform (tests were successfully run: no regression).