[py3] Let RqlIOController.publish() return bytes

This is expected by CubicWebPublisher.
We use request's encoding to encode the JSON value.

views.py

@@ -134,8 +134,8 @@ class RqlIOController(Controller):
         except Exception as exc:
             raise RemoteCallFailed(exc_message(exc, self._cw.encoding))
         if result is None:
-            return ''
-        return json_dumps(result)
+            return b''
+        return json_dumps(result).encode(self._cw.encoding)
 
     def rqlio(self, *rql_args):
         try: