Python 3: Fix Content-Length header in slice().

This prevents a http.client.IncompleteRead error in the test.

tools/wptserve/tests/functional/test_pipes.py

@@ -178,15 +178,14 @@ def handler(request, response):
         self.assertFalse(resp.info().get("X-TEST"))
         self.assertEqual(resp.read(), b"CONTENT")
 
-    @pytest.mark.xfail(sys.version_info >= (3,), reason="wptserve only works on Py2")
     def test_with_json_handler(self):
         @wptserve.handlers.json_handler
         def handler(request, response):
             return json.dumps({'data': 'PASS'})
         route = ("GET", "/test/test_pipes_2/", handler)
         self.server.router.register(*route)
         resp = self.request(route[1], query="pipe=slice(null,2)")
-        self.assertEqual(resp.read(), '"{')
+        self.assertEqual(resp.read(), b'"{')
 
     def test_slice_with_as_is_handler(self):
         resp = self.request("/test.asis", query="pipe=slice(null,2)")

tools/wptserve/wptserve/pipes.py

@@ -273,8 +273,9 @@ def slice(request, response, start, end=None):
                 (spelled "null" in a query string) to indicate the end of
                 the file.
     """
-    content = resolve_content(response)
-    response.content = content[start:end]
+    content = resolve_content(response)[start:end]
+    response.content = content
+    response.headers.set("Content-Length", len(content))
     return response