POST not working and I don't know why =(

I wrote the code as per the tutorial and didn't worked. I though that it may be something stupid that I wasn't seen, so I downloaded the code from here, replaced it and still not working.
The feedback of the POST is something like below. In Mongo I've the collection of documents with the same structure. Any idea why? Thanks from now!

{
"_id": "5aadc605bde70553ccb1fee9",
"__v": 0
}