Logstash::Json::GeneratorError issue

- Version: 2.4.0
- Operating System: Debian 8.0

After enabling `--log-as-json` option, some log messages cause Logstash to fail on writing json messages with error:

```
{"timestamp":"2016-10-10T13:45:44.245000+0000","message":"Pipeline aborted due to error","exception":"LogStash::Json::GeneratorError","backtrace":["/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.4.0-java/lib/logs
tash/json.rb:53:in `jruby_dump'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.4.0-java/lib/logstash/logging/json.rb:15:in `<<'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/cabin-0.8.1/lib/cabin/subscribe
r.rb:10:in `<<'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/cabin-0.8.1/lib/cabin/channel.rb:196:in `publish'","org/jruby/RubyHash.java:1342:in `each'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/cabin-0.8.1/lib/cabin
/channel.rb:193:in `publish'","org/jruby/ext/thread/Mutex.java:149:in `synchronize'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/cabin-0.8.1/lib/cabin/channel.rb:192:in `publish'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9
/gems/cabin-0.8.1/lib/cabin/mixins/logger.rb:113:in `_log'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/cabin-0.8.1/lib/cabin/mixins/logger.rb:90:in `log_with_level'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/cabin-0
.8.1/lib/cabin/mixins/logger.rb:75:in `warn'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.4.0-java/lib/logstash/config/mixin.rb:81:in `config_init'","org/jruby/RubyHash.java:1342:in `each'","/opt/logstash-2.4
.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.4.0-java/lib/logstash/config/mixin.rb:72:in `config_init'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.4.0-java/lib/logstash/outputs/base.rb:79:in `initialize'"
,"/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.4.0-java/lib/logstash/output_delegator.rb:74:in `register'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.4.0-java/lib/logstash/pipeline.rb:181
:in `start_workers'","org/jruby/RubyArray.java:1613:in `each'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.4.0-java/lib/logstash/pipeline.rb:181:in `start_workers'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.
9/gems/logstash-core-2.4.0-java/lib/logstash/pipeline.rb:136:in `run'","/opt/logstash-2.4.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.4.0-java/lib/logstash/agent.rb:491:in `start_pipeline'"],"level":"error"}
```

After removing `--log-as-json` option, all seems fine. First log entries were:

```
{:timestamp=>"2016-10-10T13:45:53.023000+0000", :message=>"Pipeline main started"}
{:timestamp=>"2016-10-10T13:46:07.479000+0000", :message=>"Error parsing json", :source=>"message_source", :raw=>"Traceback (most recent call last):", :exception=>#, :level=>:warn}
{:timestamp=>"2016-10-10T13:46:07.490000+0000", :message=>"Error parsing json", :source=>"message_source", :raw=>" File \"/usr/local/lib/python2.7/dist-packages/flask/app.py\", line 1475, in full_dispatch_request", :exception=>#<
LogStash::Json::ParserError: Unrecognized token 'File': was expecting ('true', 'false' or 'null')
at [Source: [B@5cbf9893; line: 1, column: 8]>, :level=>:warn}
```