magic-proxy: fix TypeError when trying to call get_uri() LP:#1944906

Currently the uri that is passed into urllib.parse.urlparse() is not
prefixed with "http(s)://" which leads urlparse() to return a wrong
scheme/netloc/path. Currently it looks like:

ParseResult(scheme='', netloc='',
  path='de.archive.ubuntu.com/ubuntu/dists/impish-backports/InRelease'
 , params='', query='', fragment='')

That's wrong. The path should look like
'ubuntu/dists/impish-backports/InRelease'.
Prefixing the 'host' header with 'http://' in case it's not there does
fix the problem.

This fixes:

Traceback (most recent call last):
  File "/usr/lib/python3.9/socketserver.py", line 683, in process_request_thread
    self.finish_request(request, client_address)
  File "/usr/lib/python3.9/socketserver.py", line 360, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/usr/lib/python3.9/socketserver.py", line 747, in __init__
    self.handle()
  File "/usr/lib/python3.9/http/server.py", line 427, in handle
    self.handle_one_request()
  File "/usr/lib/python3.9/http/server.py", line 415, in handle_one_request
    method()
  File "/home/tom/devel/livecd-rootfs/./magic-proxy", line 787, in do_GET
  File "/home/tom/devel/livecd-rootfs/./magic-proxy", line 838, in __get_request
  File "/home/tom/devel/livecd-rootfs/./magic-proxy", line 84, in get_uri
TypeError: can only concatenate str (not "NoneType") to str

(cherry picked from commit 3559153c7d)

magic-proxy

@@ -814,8 +814,14 @@ class ProxyingHTTPRequestHandler(http.server.BaseHTTPRequestHandler):
         It is important to understand that there is no status 3xx HTTP redirect
         happening here, the client does not know that what it receives is not
         exactly what it requested."""
+        host = self.headers.get("host")
+
+        # the host does not start with http(s):// which result in urlparse
+        # to not detect the host & path correctly (LP:#1944906)
+        if not host.startswith("http"):
+            host = "http://{}".format(host)
+        uri = host + self.path
 
-        uri = self.headers.get("host") + self.path
         parsed = urllib.parse.urlparse(uri)
 
         self.sanitize_requestline()