Skip missing fields, as converting those introduces new fields.
Explicitly specify fields to be ignored for GELF.
Add parameter to select the field that provides the GELF short message.
Fix spacing.
Add `short_message` to the list of fields to be ignored for GELF.
Merge branch 'master' of https://github.com/ralphm/logstash into ralphm-master

Conflicts:
	lib/logstash/outputs/gelf.rb