WebSearch: journal hint service utf8 fix

* Fixes journal hint service: decodes utf8 before passing the query
  to `search_from_reference`

* Adds two unit tests to `search_from_reference`, just to be sure that it
  accepts unicode

Signed-off-by: Federico Poli <feedrico.poli@cern.ch>
mmh... but the assumption in Invenio master is that we use strings everywhere, so how come p was a unicode object?