title: setting overrideMimeType("application/json"); with Prototype.js
body: <p>I'd like to set overrideMimeType("application/json"); when using Ajax.Request.</p>
  
  <p>As far as I can tell I think I should be using the <code>onCreate</code> option, like so:</p>
  
  <pre><code>new Ajax.Request("x.json", {onCreate: function() {...}});
  </code></pre>
  
  <p>and then set <code>overrideMimeType("application/json");</code> on the xhr, except I'm not sure how to reference the xhr from this <code>onCreate</code> function. The xhr is not <code>this</code> and it's not <code>arguments[0]</code> afaict.</p>
  
  <p>How do I set <code>overrideMimeType("application/json");</code> on the xhr?</p>