[API] Support if_primary_term param on index API

Author: estolfo

elasticsearch-api/lib/elasticsearch/api/actions/index.rb

@@ -119,7 +119,8 @@ def index(arguments={})
           :ttl,
           :version,
           :version_type,
-          :if_seq_no ].freeze)
+          :if_seq_no,
+          :if_primary_term ].freeze)
     end
   end
 end