Revert r43804 'This should probably be in Response, not Request, as we're setting...

Revert r43804 'This should probably be in Response, not Request, as we're setting data, not getting it. Nothing's using it yet (fairly new), so nothing to update.'

Session id is used by client request to specify its login data from cookie, so the session data, by extensions, is like a sort of request parameter. Also, WebResponse.php seems to be used for data actually sent to the client, like requested cookies.