Use Logger#log_warn for Aggregation and MapReduce reroute warnings.

Author: estolfo

lib/mongo/collection/view/aggregation.rb

@@ -110,7 +110,9 @@ def send_initial_query(server)
           begin
             initial_query_op.execute(server.context)
           rescue Mongo::Error::NeedPrimaryServer
-            warn 'Rerouting the Aggregation operation to the primary server.'
+            log_warn([
+              'Rerouting the Aggregation operation to the primary server.'
+            ])
             server = ServerSelector.get(mode: :primary).select_server(cluster)
             initial_query_op.execute(server.context)
           end

lib/mongo/collection/view/map_reduce.rb

@@ -177,7 +177,9 @@ def send_initial_query(server)
             begin
               initial_query_op.execute(server.context)
             rescue Mongo::Error::NeedPrimaryServer
-              warn 'Rerouting the MapReduce operation to the primary server.'
+              log_warn([
+                'Rerouting the MapReduce operation to the primary server.'
+              ])
               server = ServerSelector.get(mode: :primary).select_server(cluster)
               initial_query_op.execute(server.context)
             end