search: change union_update to union
search: change union_update to union

* Due to changes in the behaviour of intbitset union_update function,
  union should be used if a return result is expected.

Reported-by: Kamil Neczaj <f5effa4f8fb04510feb8c36db9502b05ce6bf54d@cern.ch>
Signed-off-by: Javier Martin Montull <ea6b80e26c65acc242d8916fb6eefa01b123cb69@cern.ch>

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