Fix the endless data attack issue

test_endless_data_attack.py:
Serving HTTP on 0.0.0.0 port 44631 ...
Server terminated.

Without TUF: Endless Data Attack was Successful!

Serving HTTP on 0.0.0.0 port 32722 ...
With TUF: It is maybe endlessattack!
Server terminated.


test_download.py:

test_download_url_to_tempfileobj (__main__.TestDownload) ... 
	Server process started.
	Server process id: 18327
	Serving on port: 38341
	Server process 18327 terminated.
ok

----------------------------------------------------------------------
Ran 1 test in 0.128s

OK