Set Time#iso8601 instead of Time object into record

Time object should not be member of record. It causes "undefined
method `to_msgpack`" error because Time object is not defined in
msgpack format spec.

lib/fluent/plugin/parser_groonga_log.rb

@@ -36,7 +36,11 @@ def parse(text)
           event_time = Fluent::EventTime.from_time(statistic.timestamp)
           record = {}
           statistic.each_pair do |member, value|
-            record[member.to_s] = value
+            if value.kind_of?(Time)
+              record[member.to_s] = value.iso8601
+            else
+              record[member.to_s] = value
+            end
           end
           yield event_time, record
         end