Issues with Angular 5 POST - sending data as a single value

Question

I am using this to send a POST request:

this.http.post('/auth', {uaUsername: "test", uaPassword: "test"})
    .subscribe((data: Profile) => this.profileProvider.profile = { ...data });

But in chrome network inspect, I see this being sent as a string ({"uaUsername":"test","uaPassword":"test"}), in as a key, and with a blank value, instead of 2 key/value objects

I have a interceptor that sets the header:

request = request.clone({
    setHeaders: {
        "Content-Type": "application/x-www-form-urlencoded"
    }
});

// setting the accept header
request = request.clone({ headers: request.headers.set("Accept", "application/json") });

Without the first interceptor, I saw that instead of the POST request it's doing an OPTIONS request

Any idea what could be wrong here, and why it wont send the right format ? If I use uaUsername=test&uaPassword=test it will work, but I dont want to do that

Thank you