Add support for Aggregations in Update. [DATAMONGO-2331]

[spring-projects-issues]

Christoph Strobl opened DATAMONGO-2331 and commented

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Reference URL: https://docs.mongodb.com/manual/reference/method/db.collection.update/#update-with-an-aggregation-pipeline

Issue Links:

• DATAMONGO-2363 Add support for $merge aggregation stage
  ("is depended on by")

Referenced from: pull request #789, and commits 0cc3532, 07b52d0, 1a5de2e, 476c758, cc07a1b, 46b038c, afa0b0a, 32cbae0