title: Vaadin ComboBox selects previous value instead of empty value
body: <p>I have a ComboBox, where as user types the values are loaded from a webservice. If no values are found, I want to reset the combobox to empty value. An example with fixed 2 items without using webservice:</p>
  
  <pre><code>ComboBox comboBox = new ComboBox();
  comboBox.addItem("First item");
  comboBox.addItem("Second item");
  comboBox.setNullSelectionAllowed(true);
  comboBox.setInputPrompt("Select an item:");
  comboBox.select("First item");
  </code></pre>
  
  <p>When I tried typing in input prompt "third" and then click away to loose focus, the "First item" stays selected.</p>
  
  <p>When I tried to override this behaviour by manually calling <code>comboBox.select(null)</code> or <code>comboBox.select("")</code> or even <code>comboBox.setValue(null)</code>, still the "First item" stays selected.
   
  This sound like a comboBox bug to me, but maybe somebody knows a workaround.</p>