Fixed getSpecificPortMappingEntry() null protocol.

The UPnP specification for the "GetSpecificPortMappingEntry" command does not return the "NewProtocol" key. So the protocol will always be set to null. Instead, we set the protocol using the parameter (in a similar manner to externalPort).
Merge pull request #5 from CliftonAncell/master

Fixed getSpecificPortMappingEntry() null protocol.