After saving and reading the .jay file twice, an error will be reported

**BUG:**
**when i save and read the '.jay' file twice, an error will be reported like this :**
Traceback (most recent call last):
E:/BaiduNetdiskWorkspace/01-----------------ModelDevelopment/XAJ/V2/Main.py:28 in a = fread("111.jay")
$PY/datatable\utils\fread.py:122 in _resolve_source_any return _resolve_source_file(src, tempfiles)
$PY/datatable\utils\fread.py:203 in _resolve_source_file return _resolve_archive(file, None, tempfiles)
$PY/datatable\utils\fread.py:326 in _resolve_archive out_result = core.open_jay(filename)
(with $PY = C:\Users\lenovo\AppData\Local\Programs\Python\Python36\lib\site-packages\)

IOError: Invalid signature for a Jay file: last 4 bytes are \x00\x00\x00\x00

**CODE：**
```
url = "https://raw.githubusercontent.com/Rdatatable/data.table/master/vignettes/flights14.csv"
a = fread(url)
a.to_jay("111.jay")
a = fread("111.jay")
a.to_jay("111.jay")
a = fread("111.jay")
```

**Enviroment:**
datatable version: V1.0.0
python version: 3.6.4
operating system: WIN 11

**Please help me, THANKS！**