Fix toString method of LogStash::Timestamp
According to LogStash::Timestamp (bb30cc773b/logstash-core-event/lib/logstash/timestamp.rb) doesn't support iso8601, which results in error if the timestamp of logstash is used directly.
If should support to_s of to_iso8601.
:message=>"Failed to flush outgoing items", :outgoing_count=>1, :exception=>"NoMethodError", :backtrace=>["/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-jdbc-0.2.3/lib/logstash/outputs/jdbc.rb:255:in `add_statement_event_params'", "org/jruby/RubyArray.java:1613:in `each'", "org/jruby/RubyEnumerable.java:974:in `each_with_index'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-jdbc-0.2.3/lib/logstash/outputs/jdbc.rb:251:in `add_statement_event_params'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-jdbc-0.2.3/lib/logstash/outputs/jdbc.rb:203:in `safe_flush'", "org/jruby/RubyArray.java:1613:in `each'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-jdbc-0.2.3/lib/logstash/outputs/jdbc.rb:200:in `safe_flush'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-jdbc-0.2.3/lib/logstash/outputs/jdbc.rb:120:in `flush'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/stud-0.0.22/lib/stud/buffer.rb:219:in `buffer_flush'", "org/jruby/RubyHash.java:1342:in `each'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/stud-0.0.22/lib/stud/buffer.rb:216:in `buffer_flush'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/stud-0.0.22/lib/stud/buffer.rb:159:in `buffer_receive'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-jdbc-0.2.3/lib/logstash/outputs/jdbc.rb:113:in `receive'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.0-java/lib/logstash/outputs/base.rb:83:in `multi_receive'", "org/jruby/RubyArray.java:1613:in `each'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.0-java/lib/logstash/outputs/base.rb:83:in `multi_receive'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.0-java/lib/logstash/output_delegator.rb:130:in `worker_multi_receive'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.0-java/lib/logstash/output_delegator.rb:114:in `multi_receive'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.0-java/lib/logstash/pipeline.rb:305:in `output_batch'", "org/jruby/RubyHash.java:1342:in `each'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.0-java/lib/logstash/pipeline.rb:305:in `output_batch'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.0-java/lib/logstash/pipeline.rb:236:in `worker_loop'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-2.3.0-java/lib/logstash/pipeline.rb:205:in `start_workers'"], :level=>:warn}
c4b62769b9