title: LokiJS: insert existing value for index doesn't error - how to make unique indices?
body: <p>if I try to <strong>override an existing</strong> <code>indexed</code> field, I do not get an error.
  It should error, because it is not <code>update()</code>!</p>
  
  <pre><code>var loki = require('lokijs');
  var db = new loki('test.json');
  var users = db.addCollection('users', { indices: ['userID']});
  users.insert(
     {
       'name': 'Anna',
       'userID': 1
     },
     {
       'name': 'Bernd',
       'userID': 2
     },
     {
       'name': 'Christa',
       'userID': 3
     });
  db.save();
  users.insert({'name': 'Dieter','userID': 2}); // this should error!!
  </code></pre>
  
  <p>How can I make an unique index to get an <strong>error when trying to inset an existing userID ?</strong></p>