Bug with `undefined` handling

[olessavluk]

Hello,

In this file https://github.com/comapi/comapi-sdk-js/blob/master/src/profile.ts

    /**
     * 
     * @param useEtag 
     */
    private getMyProfileETag(useEtag: boolean): Promise<string> {
        if (useEtag) {
            return this._localStorage.getString("MyProfileETag");
        } else {
            return Promise.resolve(undefined);
        }
    }

Promise<string> and Promise.resolve(undefined) are incompatible, there should be type error.

Later when this undefined saved in localStorage it is converted to a string and being sent with ETag as "undefined". This cause request to fail with HTTP error code 412