Angular4 / Ionic3 как сериализатор данных

Вам нужно будет использовать URLSearchParams

Базовый пример

let params = new URLSearchParams();
params.set('search', term); // the user's search value

Установить параметры поиска

Ваш сервис

import { Headers, RequestOptions, Http, Response, URLSearchParams } from '@angular/http';


// User is done editing, serialize and POST to web service
tokenAuthenticate(): void {
    let headers = new Headers({ 'Content-Type': 'application/x-www-form-urlencoded' });
    let options = new RequestOptions({ headers: headers });

    // Dynamically serialize the entire object
    // *** THIS IS THE SERIALIZATION ***
    let params: URLSearchParams = this.serialize(this.selectedItem);


    this._http.post('http://localhost:8080/api/oauth/token', params, options)
      .map(this.extractData)
      .catch(this.handleError);

}

/**
 * Serializes the form element so it can be passed to the back end through the url.
 * The objects properties are the keys and the objects values are the values.
 * ex: { "a":1, "b":2, "c":3 } would look like ?a=1&b=2&c=3
 * @param obj - Object to be url encoded
 * @returns URLSearchParams - The url encoded system setup
 */
serialize(obj: any): URLSearchParams {
    let params: URLSearchParams = new URLSearchParams();

    for (var key in obj) {
        if (obj.hasOwnProperty(key)) {
            var element = obj[key];

            params.set(key, element);
        }
    }
    return params;
}