通过telnet发送POST数据，发送成功了，但$_POST输出却是空的？
POST /TEST/http/http002.php HTTP/1.1
Host:localhost
Content-Type:application/x-www-form-urlencoded
Content-Length:34
username=abc808088&password=123456
-----------------------------------------
输出结果:
username=abc808088&password=123456HTTP/1.1 200 OK
Date: Tue, 17 Oct 2017 12:40:59 GMT
Server: Apache/2.4.27 (Win64) PHP/5.6.31
X-Powered-By: PHP/5.6.31
Content-Length: 182
Content-Type: text/html; charset=UTF-8
array(0) {} //此处应显示为var_dump($_POST)，结果确为空;
