title: How to custom @FeignClient Expander to convert param？
body: <p>Feign default expander to convert param：</p>
  
  <pre><code>final class ToStringExpander implements Expander {
  
      @Override
      public String expand(Object value) {
        return value.toString();
      }
    }
  </code></pre>
  
  <p>I want custom it to convert user to support <code>GET</code> param, like this</p>
  
  <pre><code>@FeignClient("xx")
  interface UserService{
  
     @RequestMapping(value="/users",method=GET)
     public List&lt;User&gt; findBy(@ModelAttribute User user);
  }
  
  userService.findBy(user);
  </code></pre>
  
  <p>What can i do?</p>