It was driving me crazy why the Http.post wasn’t working until I debugged the method itself and found that the arguments are not in the order reported in the API reference. Options and data are inverted.
It applies also to the put method.
https://developer.playcanvas.com/en/api/pc.Http.html#post
Sorry you were bitten by this documentation bug. I have committed a fix here:
This won’t update the API Reference Manual page until the next time we promote master to stable on GitHub. Hopefully some time this week.
In future, if you ever find a bug in the engine (docs or code), please do submit a GitHub issue:
I also recommend hitting the ‘Watch’ button on the Engine repo to keep up to date with all the activity there. 
Thank you. I’m really enjoying PlayCanvas.
Once I’ll understand a bit more its core I’ll be happy to contribute also by implementing new features, samples, and documentation.
That would be amazing! We always appreciate community contributions to the engine repo. 