ajax异步发送 formData数据 出错＜b＞Warning＜/b＞: Missing boundary in multipart/form-data POST data in ＜b＞Unkn

 <input type="text" name="snane" value="" id="fsnane">
 <input type="button" name="" value="发送formdata" id="btn2">

   document.getElementById('btn2').onclick=function(){
   	   
   	   var formdata=new  FormData();
   	   formdata.append('name',document.getElementById('fsnane').value);
   	   var xhr=new XMLHttpRequest();
   	   xhr.onreadystatechange=function(){
   	   	    if(xhr.readyState==4){
   	   	    	console.log(xhr.responseText);
   	   	    }
   	   }
   	   xhr.open('post','http://127.0.0.1/cheshi/0002.php');
   	   xhr.setRequestHeader("Content-Type","multipart/form-data;charset=utf-8; ");
   	   xhr.send(formdata);
   }

<?php
     header('Access-Control-Allow-Origin:*');
     //采用cors 解决了跨域的问题
     if($_POST['name']=='admin'){
     	echo 1;
     }else{
          echo '0'.$_POST['sex'];
     }
?>

会出错：

<b>Warning</b>:  Missing boundary in multipart/form-data POST data in <b>Unknown</b> on line <b>0</b><br />

把这句话去掉   xhr.setRequestHeader("Content-Type","multipart/form-data;charset=utf-8; ");

  document.getElementById('btn2').onclick=function(){
   	   
   	   var formdata=new  FormData();
   	   formdata.append('name',document.getElementById('fsnane').value);
   	   var xhr=new XMLHttpRequest();
   	   xhr.onreadystatechange=function(){
   	   	    if(xhr.readyState==4){
   	   	    	console.log(xhr.responseText);
   	   	    }
   	   }
   	   xhr.open('post','http://127.0.0.1/cheshi/0002.php');

   	   xhr.send(formdata);
   }