title: how to send payload with python mechanize
body: <p>I want to get info from internet, I use python mechanize modules to do this,
  But it need post payload while request, I trid many ways but still failed. please help me.<br>
  here is the post data i grap:</p>
  
  <h2>Request</h2>
  
  <p>URL:xxxxxx<br>
  Request Method:POST<br>
  Status Code:200 OK</p>
  
  <h1>Request Header</h1>
  
  <pre><code>**Accept:application/json, text/javascript, */*; q=0.01**  
  Accept-Charset:gb18030,utf-8;q=0.7,*;q=0.3  
  Accept-Encoding:gzip,deflate,sdch  
  Accept-Language:en-US,en;q=0.8,zh-CN;q=0.6,zh;q=0.4  
  Connection:keep-alive  
  Content-Length:52  
  **Content-Type:application/json; charset=UTF-8**  
  Cookie:ASP.NET_SessionId: sanntewiq5cz5uq1y0l5g3gy  
  Host:xxx.xxx.xxx.xxx:8500  
  Origin:xxx.xxx.xxx.xxx:8500  
  User-Agent:Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.17 (KHTML, like Gecko)  Chrome/24.0.1312.57 Safari/537.17  
  **X-Requested-With:XMLHttpRequest**
  </code></pre>
  
  <h1>Request Payload</h1>
  
  <p><strong>{dispatchorderid:"966A48E572624F2FB2E99F371C232729"}</strong></p>
  
  <h1>here is my code:</h1>
  
  <pre><code>br = mechanize.Browser()
  payload = {'dispatchorderid': "966A48E572624F2FB2E99F371C232729"}
  json_data = json.dumps(payload)
  br.addheaders = [('Content-Type', 'application/json; charset=UTF-8'),
                  ('Accept', 'application/json, text/javascript, */*; q=0.01'),
                  ('X-Requested-With', 'XMLHttpRequest')]
  br.set_debug_http(True)
  res = br.open(url, json_data)
  </code></pre>
  
  <h2>when i run this script, the send info is:</h2>
  
  <p>send: 'POST /DealerManage/AddDispatchorder.aspx/GetDispatchorderEntityList HTTP/1.1\r\nAccept-Encoding: identity\r\nContent-Length: 55<br>
  Connection: close<br>
  Accept: application/json, text/javascript, <em>/</em>; q=0.01<br>
  Host: 125.64.15.71:8500<br>
  Cookie: ASP.NET_SessionId=34dyw50d3omel2vbsvm41zwp<br>
  X-Requested-With: XMLHttpRequest<br>
  <strong>*<em>Content-Type: application/x-www-form-urlencoded'</em></strong> </p>
  
  <p><br>
  <br></p>
  
  <p>Why the Content-Type not change as 'application/json; charset=UTF-8'?</p>