async getJson(requestUrl, additionalHeaders = {}) {
        additionalHeaders[Headers.Accept] = this._getExistingOrDefaultHeader(additionalHeaders, Headers.Accept, MediaTypes.ApplicationJson);
        let res = await this.get(requestUrl, additionalHeaders);
        return this._processResponse(res, this.requestOptions);
    }
Gets a typed object from an endpoint
Be aware that not found returns a null.  Other errors (4xx, 5xx) reject the promise