title: toLocaleDateString returning String without caring of locale
body: <p>I'm developping a Discord bot in NodeJS with Discord.js, and I wanted to parse a date in format "YYYY-MM-DD" and display in long fr-FR format.</p>
  
  <p>I tried : </p>
  
  <pre class="lang-js prettyprint-override"><code>var dateSortie = new Date("2018-06-03");
  var options = {
    weekday: 'long',
    year: 'numeric',
    month: 'long',
    day: 'numeric'
  };
  console.log(dateSortie.toLocaleDateString("fr-FR", options));
  </code></pre>
  
  
  
  <p>And no matters the locale (fr-Fr, de-De, ...), it's always returning <code>"2018 M06 3, Sun"</code>.</p>