title: select2 MULTIPLE placeholder does not work
body: <p>I'm using the following piece of code to select item but placeholder already is not visible. I am using this example <a href="http://fk.github.io/select2-bootstrap-css/" rel="noreferrer">select2-bootstrap</a></p>
  
  <pre><code>   &lt;div class="form-group" style="width:70px; height:44px;"&gt;
    &lt;select class="form-control select2" multiple="multiple" id="select2" placeholder="cawky parky"&gt;
      &lt;option&gt;ahoj&lt;/option&gt;
      &lt;option&gt;more&lt;/option&gt;
      &lt;option&gt;ideme&lt;/option&gt;
      &lt;option&gt;na &lt;/option&gt;
      &lt;option&gt;pivo?&lt;/option&gt;
    &lt;/select&gt;
  &lt;/div&gt;
    &lt;script src="//select2.github.io/select2/select2-3.4.2/select2.js"&gt;&lt;/script&gt;
  
  $(document).ready(function () {
  $("#select2").select2({
      multiple:true,
      placeholder: "haha",
  });
  </code></pre>