Python: supporting ASGI legacy protocol.

Introducing manual protocol selection for 'universal' apps and frameworks.

test/python/legacy/asgi.py

@@ -0,0 +1,13 @@
+def application(scope):
+    assert scope['type'] == 'http'
+
+    return app_http
+
+async def app_http(receive, send):
+    await send({
+        'type': 'http.response.start',
+        'status': 200,
+        'headers': [
+            (b'content-length', b'0'),
+        ]
+    })