Fix inconsistent HTTPStatus string representation (#60)

In some python environments, HTTPStatus conversion to string will not result in their value but in their repr ("<HTTPStatus.OK: 200>" instead of "200")

a2wsgi/asgi.py

@@ -21,7 +21,7 @@ def __missing__(self, key):
 
 StatusStringMapping = defaultdict(
     lambda status: f"{status} Unknown Status Code",
-    {int(status): f"{status} {status.phrase}" for status in HTTPStatus},
+    {status.value: f"{status.value} {status.phrase}" for status in HTTPStatus},
 )