Remove the automatic conversion in string

[blackillzone]

When rspec check all fields, i've removed the to_s, in order to validate the type (integer/string). It's actually a problem i've get into:

• My tests were all fine with the rspec
• I've got a conflict of mapping in ElasticSearch, because i had the same entrie, but with different type (a pid in string and one in integer). The problem was one was automatically parse by the json plugin and set in integer, and the other was parsed by the kv plugin, and was set automatically in string type...

So to resolve this proble, i've just setup the possibility to verify if the type is set correctly by logstash (with a mutate, or even a simple json parse), so we can had better tests cases (and validate the type output, at least between integer and string).

The last thing i'll need to add/check, is if in the whole config, we don't have same entries with different type (and then a potential mapping conflict).