server-nodejs: upsert should only set the author and time keys upon creation

[remyla]

The upsert operation either updates an existing node or creates it if it doesn't exist.

The author and time keys are automatically set upon node creation (see specs). Right now when we use the upsert operation the author and time keys are also updated if the node already existed.

If an user which is not the author of the node updates a node, he will become the owner. We would like to avoid this, the author key has to remain unchanged upon updates as well as the time key.