gh-159044: Restore string conversion for HTTP tunnel headers

Doc/library/http.client.rst

@@ -372,6 +372,8 @@ HTTPConnection Objects
    The *headers* argument should be a mapping of extra HTTP headers to send with
    the CONNECT request.
 
+   Header values in the mapping are converted to strings before they are sent.
+
    As HTTP/1.1 is used for HTTP CONNECT tunnelling request, `as per the RFC
    <https://datatracker.ietf.org/doc/html/rfc7231#section-4.3.6>`_, a HTTP ``Host:``
    header must be provided, matching the authority-form of the request target

Lib/http/client.py

@@ -1014,8 +1014,8 @@ def _tunnel(self):
             self._http_vsn_str.encode("ascii"))
         headers = [connect]
         for header, value in self._tunnel_headers.items():
-            header_bytes = header.encode("latin-1")
-            value_bytes = value.encode("latin-1")
+            header_bytes = str(header).encode("latin-1")
+            value_bytes = str(value).encode("latin-1")
             if not _is_legal_header_name(header_bytes):
                 raise ValueError('Invalid header name %r' % (header_bytes,))
             if _is_illegal_header_value(value_bytes):

Lib/test/test_httplib.py

@@ -398,6 +398,19 @@ def test_invalid_tunnel_headers(self):
                 with self.assertRaisesRegex(ValueError, 'Invalid header'):
                     conn._tunnel()  # Called in .connect()
 
+    def test_tunnel_headers_are_converted_to_strings(self):
+        conn = client.HTTPConnection('example.com')
+        conn.set_tunnel('tunnel', headers={
+            'None-Value': None,
+            'Integer-Value': 42,
+        })
+        conn.sock = FakeSocket('HTTP/1.1 200 Connection established\r\n\r\n')
+
+        conn._tunnel()  # Called in .connect()
+
+        self.assertIn(b'None-Value: None\r\n', conn.sock.data)
+        self.assertIn(b'Integer-Value: 42\r\n', conn.sock.data)
+
     def test_invalid_tunnel_host(self):
         cases = (
             'invalid\r.host',

Misc/NEWS.d/next/Library/2026-10-11-14-06-06.gh-issue-159044.Q7mX2p.rst

@@ -0,0 +1 @@
+Restore string conversion of HTTP CONNECT tunnel header names and values.