Quick fix for properties displayed twice in UI

The upgraded front-end sends two requests to get a list of object properties,
each request asking for a different set of variables. Because our back-end was
ignoring request parameters, the same set of variables was returned twice.

This commit fixes the immediate problem, without providing the full
implementation of the new front-end feature.

See node-inspector/node-inspector#213.

@Schoonology please review. I'll fill a new issue (Enhancement) to get the proper implementation of getProperties & getInternalProperties.