NoMethodError if field @timestamp is missing

If the field `@timestamp` is missing in an event, the following error occurs:

```
NoMethodError: undefined method `to_iso8601' for nil:NilClass
to_s at /opt/logstash-2.1.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.1.0-java/lib/logstash/event.rb:109
encode at /opt/logstash-2.1.0/vendor/bundle/jruby/1.9/gems/logstash-codec-line-2.0.2/lib/logstash/codecs/line.rb:53
receive at /opt/logstash-2.1.0/vendor/bundle/jruby/1.9/gems/logstash-output-stdout-2.0.3/lib/logstash/outputs/stdout.rb:49
handle at /opt/logstash-2.1.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.1.0-java/lib/logstash/outputs/base.rb:81
output_func at (eval):47
outputworker at /opt/logstash-2.1.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.1.0-java/lib/logstash/pipeline.rb:276
start_outputs at /opt/logstash-2.1.0/vendor/bundle/jruby/1.9/gems/logstash-core-2.1.0-java/lib/logstash/pipeline.rb:193
```

**Steps to reproduce**

Start Logstash with the following config:

```
input {
generator {}
}
filter {
mutate {
rename => {
"@timestamp" => "timestamp"
}
}
}
output {
stdout {}
}
```