BibMerge: fix way to get checkbox status
BibMerge: fix way to get checkbox status

* Fixes way to get the status (checked or not) of the
  'bibMergeDupeCheckbox' checkbox.  Solves problem of
  merging record and marking slave candidate as duplicate.

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