Fix the merge function of the mutate filter

[johnmschoonover]

The dest_field_value was derived before the added fields were iterated upon. This caused the subsequent event.set's, when iterating over multiple added_field_value's, to set the dest_field based on the initial dest_field_value.

The logic should, instead, update the dest_field_value on each iteration of added_field so that it may continue to add to the dest_field_value instead of always only adding the last item in the list.

Thanks for contributing to Logstash! If you haven't already signed our CLA, here's a handy link: https://www.elastic.co/contributor-agreement/

[cla-checker-service[bot]]

💚 CLA has been signed

[johnmschoonover]

I have signed the contributor agreement

[TotalKnob]

I concluded the same and opened issue #167 and a PR including tests #168

[johnmschoonover]

Awesome, thanks much!

Pls don't hesitate to LMK if further work/assistance is needed to get this resolved :)