BibFormat: fix opensearch prefix exclusion in RSS
BibFormat: fix opensearch prefix exclusion in RSS

* Do not instruct the XSL parser to exclude 'opensearch' namespace
  prefix from RSS XSL, as this prefix is not used in the stylesheet
  and libxslt would unnecessarily report a message in apache.err.

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