search: fix pypeg2 unicode problem
search: fix pypeg2 unicode problem

* FIX Enforces query string to be unicode to overcome pypeg2 parsing
  issues.  (closes #3296)

Signed-off-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

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