fix for numeric values and avoid @target verification, some cleanups

[colinsurprenant]

Fixes #13

The following example was crashing because of numeric values

$ bin/logstash -e 'input {stdin {codec => json_lines}} filter{split{field => "a"}} output{stdout {codec => rubydebug}}'
...
{"a":[1,2,3]}

This PR fixed the numeric case and also adds an optimization to avoid checking for @target for every cloned events. Also adds corresponding specs.

[yaauie]

LGTM 👍

[yaauie]

Changelog & Version Bump please 😄

[colinsurprenant]

Ignoring unrelated build error, published 3.1.7.