Allow headers to be sent as iterables

tests/protocols/test_http.py

@@ -966,3 +966,17 @@ async def test_return_close_header(protocol_cls, close_header: bytes):
     assert b"content-type: text/plain" in protocol.transport.buffer
     assert b"content-length: 12" in protocol.transport.buffer
     assert close_header in protocol.transport.buffer
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize("protocol_cls", HTTP_PROTOCOLS)
+async def test_iterator_headers(protocol_cls):
+    async def app(scope, receive, send):
+        headers = iter([(b"x-test-header", b"test value")])
+        await send({"type": "http.response.start", "status": 200, "headers": headers})
+        await send({"type": "http.response.body", "body": b""})
+
+    protocol = get_connected_protocol(app, protocol_cls)
+    protocol.data_received(SIMPLE_GET_REQUEST)
+    await protocol.loop.run_one()
+    assert b"x-test-header: test value" in protocol.transport.buffer

uvicorn/protocols/http/h11_impl.py

@@ -468,10 +468,7 @@ async def send(self, message: "ASGISendEvent") -> None:
             self.waiting_for_100_continue = False
 
             status_code = message["status"]
-            message_headers = cast(
-                List[Tuple[bytes, bytes]], message.get("headers", [])
-            )
-            headers = self.default_headers + message_headers
+            headers = self.default_headers + list(message.get("headers", []))
 
             if CLOSE_HEADER in self.scope["headers"] and CLOSE_HEADER not in headers:
                 headers = headers + [CLOSE_HEADER]