Grok singles option is broken for complex patterns

I tried the new singles option with the patterns I'm working on and I found out that for some fields it doesn't work. I wasn't able to debug the issue since I don't know enough logstash internals, but I was able to reproduce the issue in a spec, you can find that in this branch

https://github.com/fabn/logstash/tree/singles-option-issue

The strange thing is with those data some fields are kept into arrays and for some others the single option is applied with no issues.

I did not make a pull request since in order to reproduce the issue I had to add my patterns file to the patterns directory and that file is not production ready, so you should not merge it.