add :document_counter to render_document_partial call in atom builder
so that index templates which use the 'document_counter' variable
don't cause errors in catalog#index atom-format view

From @ebenenglish
https://github.com/ebenenglish/blacklight/tree/add-doc-counter-to-atom-builder
Merge pull request #627 from projectblacklight/doc_counter_on_atom

add :document_counter to render_document_partial call in atom builder