title: WPF QuickConverter: boolean AND not working
body: <p>I'm having trouble using a multi binding in WPF <code>QuickConverter</code> where I'm using two parameters in "AND" between them:</p>
  
  <pre><code>&lt;Grid Visibility="{qc:MultiBinding '$P2 != null &amp;amp;&amp;amp; $P1 == OutputTarget.Disk ? Visibility.Visible : Visibility.Hidden', P1={Binding Target}, P2={Binding ElementName=uc1, Path=SelectedOutput}}"&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p>Note that if I use only one of them it works! For example, this:</p>
  
  <pre><code>&lt;Grid Visibility="{qc:MultiBinding '$P2 != null ? Visibility.Visible : Visibility.Hidden', P2={Binding ElementName=uc1, Path=SelectedOutput}}"&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p>works and so does this:</p>
  
  <pre><code>&lt;Grid Visibility="{qc:MultiBinding '$P1 == OutputTarget.Disk ? Visibility.Visible : Visibility.Hidden', P1={Binding Target}}"&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p>So why is the one with the two expressions in AND not working? Note that I tried boht <code>&amp;amp;&amp;amp;</code> and <code>##</code> but the result doesn't change
  Also note that I've correctly registered all types involved in the expression.</p>