title: ngx-translate custom loader doesn't find translations
body: <p>I decided to implement custom loader for translations because I have an issue that string want to get translated before loader loads correct translations. I followed example on ngx-translate github but now everywhere I get keys instead of values. My custom loader: </p>
  
  <pre><code>    class CustomLoader implements TranslateLoader {
    getTranslation(lang: string): Observable&lt;any&gt; {
      return Observable.of({ KEY: 'value' });
    }
  }
  </code></pre>
  
  <p>in app.module imports I have: </p>
  
  <pre><code>    TranslateModule.forRoot({
    loader: { provide: TranslateLoader, useClass: CustomLoader }
  })
  </code></pre>
  
  <p>My translations are in ./assets/i18n/  and are all json. Anyone know what I need to change so loader will find my translations?</p>