BaseHTTPServer 写了个简易 HTTP 服务器，但是有时候 Response 没有头部，求大佬指点！

就是一个很简单的 HTTP 服务器，响应任意 GET 请求，返回一个文件
服务端用 clumsy 模拟乱序环境，乱序 10%
然后在客户端抓包发现有时候服务端的 Response 没有 HTTP 头，不知道为什么……
是 python 有 bug 还是我写的代码有问题？求大佬指点

有头部的流

GET /122879/122879.bat HTTP/1.1
Host: 200.200.89.77:8832
User-Agent: python-requests/2.18.4
Accept-Encoding: gzip, deflate
Accept: */*
Connection: keep-alive

HTTP/1.0 200 OK
Server: BaseHTTP/0.6 Python/3.7.0
Date: Fri, 03 Aug 2018 15:10:09 GMT
Content-Length: 252416
Last-Modified: Thu, 14 Sep 2017 05:05:10 GMT

MZ......................@.............................................	.!..L.!This program cannot be run in DOS mode.

$.......PE..L.....:1.................x...^......

没有头部的流

GET /122877/122877.bat HTTP/1.1
Host: 200.200.89.77:8832
User-Agent: python-requests/2.18.4
Accept-Encoding: gzip, deflate
Accept: */*
Connection: keep-alive

MZ......................@.............................................	.!..L.!This program cannot be run in DOS mode.

$.......PE..L.....:1.................x...^......

源代码

from http.server import BaseHTTPRequestHandler
from http.server import HTTPServer
import datetime
import os
import shutil

# Listen Address
ADDR = ''
# Listen Port
PORT = 8832

class WebRequestHandler(BaseHTTPRequestHandler):
    def do_GET(self):
        path = "./testb"
        try:
            f = open(path, 'rb')
        except IOError:
            self.send_error(404, "File not found")
            return None
        self.send_response(200)
        #self.send_header("Content-type", 'image/png')
        #self.send_header('Content-Disposition','attachment; filename=test.exe')
        fs = os.fstat(f.fileno())
        self.send_header("Content-Length", str(fs[6]))
        self.send_header("Last-Modified", self.date_time_string(fs.st_mtime))
        self.end_headers()  
        shutil.copyfileobj(f, self.wfile)
        f.close()

server = HTTPServer((ADDR, PORT), WebRequestHandler)
print("Server start!")
server.serve_forever()