Grok does create field when pattern matches all message

As of today grok filter does not create a specific field
if the grok matches the whole message. This commit changes
this behavior and make grok create the required field in
the configuration file
Merge branch 'master' of github.com:Spredzy/logstash into Spredzy-master

Conflicts:
	spec/filters/grok.rb
Avoid multiple subscriptions
- clone the message string; otherwise the test suite for the kv filter
  fires this warning: "RuntimeError: string modified"
- fix failing test