title: Object to string ValueConverter not getting invoked
body: <p>I have defined a Value Converter in my PCL like this:</p>
  
  <pre><code>class EmployeeToStringValueConverter : MvxValueConverter&lt;Employee, string&gt;
  {
      protected override string Convert(Employee value, Type targetType, object parameter, CultureInfo culture)
      {
          return value.ToString();
      }
  }
  </code></pre>
  
  <p>And I'm using it in an Android .axml layout like this:</p>
  
  <pre><code>local:MvxBind="Text EmployeeToString(SearchResult)"
  </code></pre>
  
  <p>(<code>SearchResult</code> is defined in the ViewModel as a public <code>Employee</code> property)</p>
  
  <p>But it is not working (meaning: if I put a breakpoint in the Converter call, it is never executed).</p>
  
  <p>However, I've also defined the following converter:</p>
  
  <pre><code>public class NegateBoolValueConverter : MvxValueConverter&lt;bool, bool&gt;
  {
      protected override bool Convert(bool value, Type targetType, object parameter, CultureInfo culture)
      {
          return !value;
      }
  }
  </code></pre>
  
  <p>And I'm using it like so:</p>
  
  <pre><code>local:MvxBind="Enabled NegateBool(IsLoggedIn)"
  </code></pre>
  
  <p>(<code>IsLoggedIn</code> is a <code>bool</code> public property in the ViewModel)</p>
  
  <p>And this works perfectly.. Any idea so as to what may be going on with the first one that's not working?</p>