Select only key field in mongo.find function (for findKeys)

If you don't select only the key field, you can exceed the maxBsonSize very fast in your response => you get a null response.

I get the problem with 1057 pads. Asking only for the key field made it works again.
Merge pull request #50 from ldidry/master

Select only key field in mongo.find function (for findKeys)