urlutils: redirect_to_url() user agent fix
urlutils: redirect_to_url() user agent fix

* Commit 49f69d35b34d80429bf87686a1ff8de4e245bbc5 introduced
  a work-around for the MS-Office User-Agent misbehaviour.
  The check was assuming the User-Agent header is always available
  in an HTTP request. This is a wrong assumption which is now
  fixed.

inveniosoftware/invenio,tiborsimko/invenio,inveniosoftware/invenio,tiborsimko/invenio