def remove_invalid_keys(solr_doc)
      valid_keys = schema_keys
      solr_doc.delete_if { |key, _| !key.in? valid_keys }
    end
def remove_invalid_keys(solr_doc)
      valid_keys = schema_keys
      solr_doc.delete_if { |key, _| !key.in? valid_keys }
    end
Remove keys (ie. fields) that are not in the Solr schema.
 @param [Hash]
 @return [Hash]