Get and Set Configuration need fixing

[arlolra]

See 9c40f4d

SetConfiguration pretty much always returns INVALID_MODIFICATION because of the following, from webrtc/pc/peerconnection.cc,

  // The simplest (and most future-compatible) way to tell if the config was
  // modified in an invalid way is to copy each property we do support
  // modifying, then use operator==. There are far more properties we don't
  // support modifying than those we do, and more could be added.
  RTCConfiguration modified_config = configuration_;
  modified_config.servers = configuration.servers;
  modified_config.type = configuration.type;
  modified_config.ice_candidate_pool_size =
      configuration.ice_candidate_pool_size;
  modified_config.prune_turn_ports = configuration.prune_turn_ports;
  modified_config.ice_check_min_interval = configuration.ice_check_min_interval;
  if (configuration != modified_config) {
    LOG(LS_ERROR) << "Modifying the configuration in an unsupported way.";
    return SafeSetError(RTCErrorType::INVALID_MODIFICATION, error);
  }

GetConfiguration needs to return more than just the go object so that can be called with a complete set of values.