Add output_field to preserved_order CASE

[dhvcc]

Hi, this PR adds output_field to ensure everything's right

I'm not really sure how nobody encountered this, but without specifying output_field I was getting errors while annotating. Here's my code:

In [3]: BookDocument.search().query("fuzzy", title='qwerty').to_queryset().annotate(a=ArrayAgg("person_relations__per
   ...: son__name"))
... ElasticsearchWarning: Elasticsearch built-in security features are not enabled. ..
Out[3]: <repr-error 'Cannot resolve expression type, unknown output_field'>

In [4]: Book.objects.filter(title='qwerty').annotate(a=ArrayAgg("person_relations__person__name"))
Out[4]: <QuerySet []>

In [5]: BookDocument.search().query("fuzzy", title='qwerty').to_queryset().annotate(a=F("person_relations__person__na
   ...: me"))
Out[5]: <QuerySet []>

Notice, how this only happens when using ArrayAgg aggregation from django.contrib.postgres.aggregates

Deps

• django: 3.1
• django-elasticsearch-dsl: 7.2.0
• elasticsearch: 7.15.1
• elasticsearch-dsl: 7.4.0
• (docker)postgres: 12

[dhvcc]

May be it has something to do with the way ArrayAgg treats other annotations, but I'm not that interested in going that deep

[codecov-commenter]

Codecov Report

Merging #381 (8c3784a) into master (1b33fb0) will decrease coverage by 0.13%.
The diff coverage is 100.00%.

❗ Current head 8c3784a differs from pull request most recent head 53598a3. Consider uploading reports for the commit 53598a3 to get more accurate results

@@            Coverage Diff             @@
##           master     #381      +/-   ##
==========================================
- Coverage   67.81%   67.67%   -0.14%     
==========================================
  Files          12       12              
  Lines         755      758       +3     
==========================================
+ Hits          512      513       +1     
- Misses        243      245       +2     

Impacted Files	Coverage Δ
django_elasticsearch_dsl/search.py	81.48% <100.00%> (-6.02%)	⬇️

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[safwanrahman]

Can you share the SQL Query it is making? So it is easier to undersrtand

[safwanrahman]

I think it can be merged as it does not break any current functionality. It is better to be explicit

[dhvcc]

@safwanrahman Hey, sorry I didn't have time to provide the SQL part, but basically it's the same SQL as usual
Thanks for merging!