UDP listener died, GELF output with collectd thresholds.

Hi!

I am experiencing a crash of logstash 1.5.4 when it is about to output events via gelf to the graylog server (1.1.6-1). The message generator is collectd 5.5.0 which sends events to logstash using network/threshold mechanism.

The cause is as follows:

NoMethodError: undefined method `downcase' for 1:Fixnum
receive at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-gelf-1.0.0/lib/logstash/outputs/gelf.rb:200
handle at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-1.5.4-java/lib/logstash/outputs/base.rb:88
output_func at (eval):123
outputworker at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-1.5.4-java/lib/logstash/pipeline.rb:244
start_outputs at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-1.5.4-java/lib/logstash/pipeline.rb:166
UDP listener died {:exception=>#, :backtrace=>["/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-input-udp-1.0.0/lib/logstash/inputs/udp.rb:79:in`udp_listener'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-input-udp-1.0.0/lib/logstash/inputs/udp.rb:49:in `run'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-1.5.4-java/lib/logstash/pipeline.rb:177:in`inputworker'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-1.5.4-java/lib/logstash/pipeline.rb:171:in `start_input'"], :level=>:warn}

This problem didn't exist in Logstash version 1.4.5-1 which I have tested.
Commenting out the gelf section below prevents Logstash from crashing.

Logstash config:

```
input {
udp {
add_field => { "short_message" => "collectd" }
port => 25826
codec => collectd {}
buffer_size => 1400
type => "linux-collectd"
}
}

output {
stdout
{
codec => rubydebug
}
gelf {
host => "10.12.21.10"
port => 12201
}
}
```

collectd config:

```

FSType "ext4"
FSType "xfs"
ValuesPercentage True

Server "127.0.0.1" "25826"

Instance "*"
WarningMin 20
FailureMin 10
Instance "free"
DataSource "value"

```

Start at logstash-output-gelf-1.0.0/lib/logstash/outputs/gelf.rb:200 and inspect the value passed to downcase when collectd threshold events are sent. Compare the UDP listener path in logstash-input-udp-1.0.0/lib/logstash/inputs/udp.rb:79, then reproduce with the provided Logstash and collectd configurations. Done means threshold events reach the GELF output without crashing Logstash or killing the UDP listener.

Written by the indexing model from the issue text.