title: getClosestFeatureToCoordinate does not work for Points
body: <p>I made a function to get the closer feature to the one clicked. I use Openlayers 3.9.0 and the <code>getClosestFeatureToCoordinate</code> method. </p>
  
  <pre><code>var select = new ol.interaction.Select();//simple click interaction
   map.addInteraction(select);//add it to the map
  
   select.on('select', function(e) {
  
     //get the extent of the first selected feature 
      var aa = e.selected[0].getGeometry().getExtent();
  
     //in case of line or polygon get the center of that extent 
      var oo = ol.extent.getCenter(aa);
  
     //use it to get the name of the closest feature
     console.log((sourceVector.getClosestFeatureToCoordinate(oo)).get("mylayer_name")) ;
   });
  </code></pre>
  
  <p>But in a case like the following</p>
  
  <p><a href="https://i.stack.imgur.com/ej2YA.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ej2YA.png" alt="enter image description here"></a></p>
  
  <p>if I click the "u" Polygon (bottom down) I get "u" instead of , say, "e"
  if I click any point I get its name , instead of the closest feature's name. I click "testpoint9" and I get "testpoint9" instead of "u" or "e". I click "h" and I get "h" instead of "p" or "k".</p>
  
  <p>So maybe has to do with points, so I changed the <code>select.on</code> event function to </p>
  
  <pre><code>select.on('select', function(e) {
      var closestType = e.selected[0].getGeometry().getType();
      var oo;
      if (closestType === 'Point'){
          oo = e.selected[0].getGeometry().getCoordinates();
      }
      else{
          var aa = e.selected[0].getGeometry().getExtent();
          oo = ol.extent.getCenter(aa);
      }
  
      console.log("---------------------------------------------------");
      console.log("Name: "+sourceVector.getClosestFeatureToCoordinate(oo).get('mylayer_name'));
  })
  </code></pre>
  
  <p>and still nothing. So, how I fix this? </p>
  
  <p>Thanks</p>