Improve documentation & tests for http.js and handle 204 code

I'd spotted this whilst working on other things, although the piece of code originally at issue is not present any more. I think it is still worth improving code & documentation though. This PR:

- Adds documentation for the callback parameters
- Handles the 204 response from the server as a success parameter (this is a simple case that we already have in Talkilla, we just don't handle errors from the server yet)
- Adds tests to get coverage of http.js to 100% - the var line I removed is because it wasn't necessary and it was confusing the coverage analysis