Correct way to pass headers to the Httpheaders Object

Question

I’m trying to figure out how to pass headers the correct way to instantiate an object HttpHeaders in the Angular 5. I have the following code:

postApi(body: any, route: string) {
   return new Promise((resolve, reject) => {      
      this.http.post(url_api + route, JSON.stringify(body), this.options)
        .subscribe(res => {
           resolve(res);
        }, (err) => {
          console.log(err)
          reject(err);
        });
    });
}

When I try to instantiate the object HttpHeaders passing an object as specified in documentation. I get a 404 return (Actually the request was made for my API. But for some reason or error it does not recognize the route):

options = {
    headers: new HttpHeaders({
      'Content-Type': 'application/json',
      'x-api-key': 'xxxxxxxxxxxxxxxxxxxx'
    })
  };

However, if I create an empty object and then add my headers with append. Everything works perfectly and my route is found without any problem:

options = {
    headers: new HttpHeaders()
  };

postApi(body: any, route: string) {
    this.options.headers.append('Content-Type', 'application/json');
    this.options.headers.append('x-api-key', `xxxxxxxxxxxxxxxxxxxx`);
...
}

So I’d like to understand if it’s a mistake instantiating an Httpheaders object like this:

options = {
    headers: new HttpHeaders({
      'Content-Type': 'application/json',
      'x-api-key': 'xxxxxxxxxxxxxxxxxxxx'
    })
  };