HTTP headers are never sent with the HTTP Client API

[mathieuancelin]

When using the HTTP Client API from a JS plugin like this:

const task = {
  "userId":1, 
  "id":2345,
  "title":"title",
  "completed":false
};
const request = {
  method: "POST",
  url: "https://jsonplaceholder.typicode.com/todos",
  headers: {
    "Accept": "application/json",
    "Content-Type": "application/json",
  },
};

const response = Http.request(
  request, 
  JSON.stringify(task),
);

it appears that the headers were never sent, hence the target server not being able to parse the body correctly and creating an empty task.

This pull request fix this behavior

[zshipko]

Thanks!