Fix how a field is fetched in AggregateJsonWriter

In the class AggregateJsonWriter, a non system field is retrieved with

            field = schemaManager.getField(fieldName);

However fieldName is a the sanitized version of the field defined on the aggregate.
Because of AggregateEsBase#getField, the "/" character is replaced by "." to have an ES-compatible field name.

But the schemaManager cannot retrieve a field from this sanitized name.

The following warn is then logged

Could not resolve field file:content.mime-type for aggregate system_mimetype_agg