Fix logging of incorrect status codes for disconnected requests (#12580)

The status code of requests must always be set, regardless of client
disconnection, otherwise they will always be logged as 200!.

Broken for `respond_with_json` in
f48792eec4.
Broken for `respond_with_json_bytes` in
3e58ce72b4.
Broken for `respond_with_html_bytes` in
ea26e9a98b.

Signed-off-by: Sean Quah <seanq@element.io>

tests/handlers/test_saml.py

@@ -349,4 +349,16 @@ class SamlHandlerTestCase(HomeserverTestCase):
 
 def _mock_request():
     """Returns a mock which will stand in as a SynapseRequest"""
-    return Mock(spec=["getClientIP", "getHeader", "_disconnected"])
+    mock = Mock(
+        spec=[
+            "finish",
+            "getClientIP",
+            "getHeader",
+            "setHeader",
+            "setResponseCode",
+            "write",
+        ]
+    )
+    # `_disconnected` musn't be another `Mock`, otherwise it will be truthy.
+    mock._disconnected = False
+    return mock