WebSearch: fixed mrss namespace
WebSearch: fixed mrss namespace

* Added trailing slash to mrss namespace to comply with latest specs.

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