Update js/jquery.smartWizard.js

When using the contentURLData parameter, the extra data was not being passed in the ajax query.
I found that this was because $.extend was adding the data returned by the function to the ajax_args object instead of ajax_args.data.

Here is example usage now with this commit : 
$("#wizard").smartWizard({
  contentURL : "my.ajax.php"
  , contentURLData : function(step) {
      if (step == 1) return {"other_arg" : 1};
      if (step == 2) return {"this_arg" : "blue"};
      return {"operation" : "default parameter value"};
    }
});

In my.ajax.php

<?php
// simple example of reading ajax query parameters
// step_number set by Smart Wizard
if ($_REQUEST["step_number"] == 1)
{
  // other_arg set by contentURLData function
  echo "Other Argument value is ".$_REQUEST["other_arg"];
}
exit;
?>