Prior to this change the `target_field` would always be a json array
field in the document being ingested. This to take into account that
multiple enrich documents could be inserted into the `target_field`.
However the default `max_matches` is `1`. Meaning that by default
only a single enrich document would be added to `target_field` json
array field.
This commit changes this; if `max_matches` is set to `1` then the single
document would be added as a json object to the `target_field` and
if it is configured to a higher value then the enrich documents will be
added as a json array (even if a single enrich document happens to be
enriched).
Martijn van Groningen
James Rodewig