BibRank: fix citedby/refersto for infinite sets
BibRank: fix citedby/refersto for infinite sets

* Fixes citedby/refersto search functions for infinite sets by ignoring
  attempts to iterate over infinite sets.  (closes #681)

* Adds regression tests to avoid this from happening in the future.

* Cherry-picked from 92d74c3a790c89f3c168fc42cac40444400b9601.

inveniosoftware/invenio-search,tiborsimko/invenio-search,inveniosoftware/invenio-search,tiborsimko/invenio-search,inveniosoftware/invenio-records,inveniosoftware/invenio-records,tiborsimko/invenio-records,tiborsimko/invenio-records