Preserve explicit Connection header casing

h11/_connection.py

@@ -647,9 +647,10 @@ def _clean_up_response_headers_for_sending(self, response: Response) -> Response
         if not self._cstate.keep_alive or need_close:
             # Make sure Connection: close is set
             connection = set(get_comma_header(headers, b"connection"))
-            connection.discard(b"keep-alive")
-            connection.add(b"close")
-            headers = set_comma_header(headers, b"connection", sorted(connection))
+            if b"close" not in connection or b"keep-alive" in connection:
+                connection.discard(b"keep-alive")
+                connection.add(b"close")
+                headers = set_comma_header(headers, b"connection", sorted(connection))
 
         return Response(
             headers=headers,

h11/tests/test_connection.py

@@ -365,6 +365,23 @@ def test_automagic_connection_close_handling() -> None:
         assert conn.states == {CLIENT: MUST_CLOSE, SERVER: MUST_CLOSE}
 
 
+def test_explicit_connection_close_preserves_header_case() -> None:
+    c = Connection(SERVER)
+    c.receive_data(
+        b"GET / HTTP/1.1\r\n"
+        b"Host: example.com\r\n"
+        b"Connection: close\r\n"
+        b"\r\n"
+    )
+    assert isinstance(c.next_event(), Request)
+    assert isinstance(c.next_event(), EndOfMessage)
+
+    assert (
+        c.send(Response(status_code=204, headers=[("connection", "close")]))
+        == b"HTTP/1.1 204 \r\nconnection: close\r\n\r\n"
+    )
+
+
 def test_100_continue() -> None:
     def setup() -> ConnectionPair:
         p = ConnectionPair()

newsfragments/156.bugfix.rst

@@ -0,0 +1,2 @@
+Preserve an explicitly provided ``Connection`` header's original casing when
+it already requests ``close``.