Explicitly enable chunked encoding for test

Author: asvetlov

tests/test_web_functional.py

@@ -889,6 +889,7 @@ def test_stream_response_multiple_chunks(self):
         @asyncio.coroutine
         def handler(request):
             resp = web.StreamResponse()
+            resp.enable_chunked_encoding()
             resp.start(request)
             resp.write(b'x')
             resp.write(b'y')