[3.7] No Content-Length header for 1xx 204 (#5119) (#5120)

Backports the following commits to 3.7:
 - No Content-Length header for 1xx 204 (#5119)

Co-authored-by: Dmitry Erlikh <derlih@gmail.com>

CHANGES/4901.bugfix

@@ -0,0 +1 @@
+Server doesn't send Content-Length for 1xx or 204

aiohttp/web_response.py

@@ -427,6 +427,10 @@ async def _prepare_headers(self) -> None:
                         del headers[hdrs.CONTENT_LENGTH]
                 else:
                     keep_alive = False
+            # HTTP 1.1: https://tools.ietf.org/html/rfc7230#section-3.3.2
+            # HTTP 1.0: https://tools.ietf.org/html/rfc1945#section-10.4
+            elif version >= HttpVersion11 and self.status in (100, 101, 102, 103, 204):
+                del headers[hdrs.CONTENT_LENGTH]
 
         headers.setdefault(hdrs.CONTENT_TYPE, "application/octet-stream")
         headers.setdefault(hdrs.DATE, rfc822_formatted_time())

tests/test_web_functional.py

@@ -13,6 +13,7 @@
 
 import aiohttp
 from aiohttp import FormData, HttpVersion10, HttpVersion11, TraceConfig, multipart, web
+from aiohttp.hdrs import CONTENT_LENGTH, TRANSFER_ENCODING
 from aiohttp.test_utils import make_mocked_coro
 
 try:
@@ -1948,3 +1949,20 @@ async def handler(request):
     resp = await client.post("/", data=b"data")
     assert resp.status == 200
     assert await resp.text() == "data (2, 4)"
+
+
+@pytest.mark.parametrize(
+    "status", [101, 204],
+)
+async def test_response_101_204_no_content_length_http11(
+    status, aiohttp_client
+) -> None:
+    async def handler(_):
+        return web.Response(status=status)
+
+    app = web.Application()
+    app.router.add_get("/", handler)
+    client = await aiohttp_client(app, version="1.1")
+    resp = await client.get("/")
+    assert CONTENT_LENGTH not in resp.headers
+    assert TRANSFER_ENCODING not in resp.headers