Stopping the server reader can cause an exception when reading.

Beats input: unhandled exception {:exception=>#, :backtrace=>["/Users/ph/es/logstash/lib/logstash/json.rb:41:in `jruby_load'", "/Users/ph/es/logstash/lib/logstash/json.rb:38:in`jruby_load'", "/Users/ph/es/ruby-lumberjack/lib/lumberjack/server.rb:246:in `json_data_payload'", "/Users/ph/es/ruby-lumberjack/lib/lumberjack/server.rb:163:in`feed'", "/Users/ph/es/ruby-lumberjack/lib/lumberjack/server.rb:330:in `read_socket'", "/Users/ph/es/ruby-lumberjack/lib/lumberjack/server.rb:315:in`run'", "/Users/ph/es/logstash-plugins/logstash-input-beats/lib/logstash/inputs/beats.rb:149:in `invoke'", "org/jruby/RubyProc.java:271:in`call'", "/Users/ph/es/logstash/vendor/bundle/jruby/1.9/gems/concurrent-ruby-0.9.1-java/lib/concurrent/executor/executor_service.rb:515:in `run'", "Concurrent$$JavaExecutorService$$Job_310320422.gen:13:in`run'"], :level=>:error}