Default the Content-Length response header to 0¬

If the response payload doesn't contain a 'Content-Length' header and
the '--ignore-content-length' parameter is provided, the script was
trying to cast a None type into an integer.

This commit fixes that by return '0' has the 'Content-Length' if
the header isn't present.

Author: diogoosorio

lib/core/virtual_host_scanner.py

@@ -134,7 +134,7 @@ def scan(self):
             if res.status_code in self.ignore_http_codes:
                 continue
 
-            response_length = int(res.headers.get('content-length'))
+            response_length = int(res.headers.get('content-length', 0))
             if self.ignore_content_length and \
                self.ignore_content_length == response_length:
                 continue