Fix default to 'index' action in node resource when nid = 0

When a call is made to the node resource and the nid = 0, i.e. http://domain/endpoint/node/0.json, the index action is automatically selected. This should be handled by the 'retrieve' action in the node resource rather than 'index' by default and trigger an exception via services_error(). This patch allows for the nid to be 0, and allows it to be added to the $path array.