title: JQuery Validation Uncaught typeError when applying custom message to remote validation
body: <p>I am trying to validate remotely whether a string already exists on the server using the <code>jqueryvalidation.org plugin.</code>  </p>
  
  <p><strong>Here is my jQuery code:</strong></p>
  
  <pre><code>$("#title").rules("add", {
      "remote": {
          url: checkTitleAvailable.php,    // echoes "true" or "false"
          type: "post",
          data: {
              proposedSeoTitle:function() {
                  return $('#title').val();
              }
          },
          complete: function(response) {
              if (response.responseText != "true" &amp;&amp; response.responseText != "false") {
                  alert("unexpected response " + response.responseText);
              }
              return response;
          }
      },
      "messages": {"remote": "That title already exists"}
  });
  </code></pre>
  
  <p>('title' is an input field with id="title" class="required")</p>
  
  <p><strong>I get an error:</strong> </p>
  
  <blockquote>
    <p>"Uncaught TypeError: Cannot call method 'call' of undefined".</p>
  </blockquote>
  
  <p><code>"messages"</code> gets treated as another rule, but there is no function to run. Yet it still gets treated as a message, and tends to pop up when it should, but the error makes the whole system unusable - it is impossible to validate the whole page before saving it.</p>
  
  <p>Am I doing something wrong, or is this a bug within the Validation plugin? Is there some workaround? Should I be looking to hack the plugin to get it working?</p>