use fieldref to properly assign event field data

[colinsurprenant]

this change is necessary to support the future Java Event.

The previous construct

event[@target][key] = foo

is actually: 1- fetch field reference @target and return value, 2- create key in value and assign foo.
this actually assumes the mutability of event[@target] by its reference, which work but is an actual implementation side-effect and not a contract.

The correct way to assign Event field values is by using proper field references

event["[#{@target}][#{key}]"]

This relates to elastic/logstash#4264 and elastic/logstash#4191

[jsvd]

LGTM

[colinsurprenant]

thanks, will bump version and merge!

[colinsurprenant]

pushed version 2.0.5