Add an xfailing test expecting HTTP500 for invalid handler return values

PR #4574 by @Fogapod

This change documents a contract of disallowing non-response
return values in request handlers that currently has a bug and
is therefore marked as xfail.

Ref: #4572

Author: Fogapod

tests/test_web_request.py

@@ -10,6 +10,7 @@
 from yarl import URL
 
 from aiohttp import HttpVersion, web
+from aiohttp.client_exceptions import ServerDisconnectedError
 from aiohttp.helpers import DEBUG
 from aiohttp.http_parser import RawRequestMessage
 from aiohttp.streams import StreamReader
@@ -771,3 +772,19 @@ async def handler(request):
 def test_weakref_creation() -> None:
     req = make_mocked_request('GET', '/')
     weakref.ref(req)
+
+
+@pytest.mark.xfail(
+    raises=ServerDisconnectedError,
+    reason="see https://github.com/aio-libs/aiohttp/issues/4572"
+)
+async def test_handler_return_type(aiohttp_client) -> None:
+    async def invalid_handler_1(request):
+        return 1
+
+    app = web.Application()
+    app.router.add_get('/1', invalid_handler_1)
+    client = await aiohttp_client(app)
+
+    async with client.get('/1') as resp:
+        assert 500 == resp.status