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.

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