Permanent Content-type duplication in response after 404

[antitree]

Description
In the case of a general 404 trying to access a file that doesn't exist, Cling incorrectly adds an additional Content-type: text/plain after every 404 received. This means that if the server sends 3 404 responses, the header will provide 3 Content-type: text/plain headers. For 10,000 responses, 10,000 duplicate headers.

This is persistent until the Headers variable is cleared or the service is restarted.

Fix
I believe that in the LOC below, self.message is never False so in every case, it will append another value.

static3/static.py

Line 109 in 309ddc3

if self.message:

POC
Run a server

from static import Cling
from wsgiref.simple_server import make_server
my_app = Cling("/home/antitree/")
make_server("localhost", 9999, my_app).serve_forever()

Connect to a file that doesn't exist
wget --server-response http://localhost:9999/TEST

Response

Connecting to 127.0.0.1:9999... connected.
HTTP request sent, awaiting response... 
  HTTP/1.0 404 Not Found
  Date: Sun, 07 Aug 2016 18:04:28 GMT
  Server: WSGIServer/0.2 CPython/3.5.2
  Content-type: text/plain
  Content-Length: 13
  Content-type: text/plain

Repeat, and second response

Connecting to 127.0.0.1:9999... connected.
HTTP request sent, awaiting response... 
  HTTP/1.0 404 Not Found
  Date: Sun, 07 Aug 2016 18:04:28 GMT
  Server: WSGIServer/0.2 CPython/3.5.2
  Content-type: text/plain
  Content-Length: 13
  Content-type: text/plain
  Content-type: text/plain

Repeat 10,000 times to see that the headers are repeatedly added to.