title: Mule Choice Expression - Routing based off Outbound property
body: <p>I have a property I am extracting from the Message's Payload and setting into an Outbound property (to make it easier to access).  I want to simply check the size of this property, but all my best guesses are giving me exceptions.</p>
  
  <p>I've tried using the #[header:OUTBOUND:count] syntax, adding an evaluator="header" attribute to the &lt;when&gt; tag, nothing seems to be working.  I've tried other evaluators as well, but I cannot find any Mule documentation with examples of syntax, so I'm just guessing how the expressions should be formed.</p>
  
  <pre><code>&lt;choice doc:name="Choice"&gt;
          &lt;when expression="message.outboundProperties['count']==0"&gt;
              &lt;processor-chain&gt;
                  ... something here ...
              &lt;/processor-chain&gt;
          &lt;/when&gt;
          &lt;otherwise&gt;
              &lt;processor-chain&gt;
                  ... alternate option ...
              &lt;/processor-chain&gt;
          &lt;/otherwise&gt;
      &lt;/choice&gt;
  </code></pre>