title: How to change the extraParams config of a proxy at runtime in extjs?
body: <p>I have the following store:</p>
  
  <pre><code>var store = new Ext.data.Store({
       model: 'Result',
       proxy: {
           type: 'ajax',
           extraParams: {search_term : term},
           url : 'find.pl'
       },
  });
  </code></pre>
  
  <p>How can I change the parameters with which the url is called (ex. search_term) at runtime?</p>