Don't explode when processing multiline events

I don't know ruby, there is likely a better solution for this, but it's a nasty problem that needs fixing.

This will allow processing of multiline events, otherwise you get

NoMethodError: undefined method `tv_sec' for #<Array:0x7f8e1a98>
        sprintf at /opt/logstash/lib/logstash/event.rb:223
           gsub at org/jruby/RubyString.java:3041
        sprintf at /opt/logstash/lib/logstash/event.rb:209
        receive at /opt/logstash/lib/logstash/outputs/elasticsearch.rb:324
         handle at /opt/logstash/lib/logstash/outputs/base.rb:86
     initialize at (eval):20
           call at org/jruby/RubyProc.java:271
         output at /opt/logstash/lib/logstash/pipeline.rb:266
   outputworker at /opt/logstash/lib/logstash/pipeline.rb:225
  start_outputs at /opt/logstash/lib/logstash/pipeline.rb:152


On my installation this error was getting swallowed entirely until I ran a debug config with a stdout {} section in output.  We'd been silently failing on all multiline events.