fix: handling of params.mongoose in transaction

[florianbepunkt]

extend params.mongoose with session instead of replacing it

Summary

Currently beginTransaction overrides the params.mongoose object:
context.params.mongoose = { session };

This causes issues if an operations depends on mongoose/mongodb params, for example when using arrayFilters or upsert (https://docs.mongodb.com/master/reference/method/db.collection.findAndModify/#findandmodify-arrayfilters)

Not sure if there was an intentional design decision behind overwriting it. If so, some sort of whitelisting for options should be included in my opinion.

• Tell us about the problem your pull request is solving.
  See desc
• Are there any open issues that are related to this?
  No, I just fixed it instead of opening one
• Is this PR dependent on PRs in other repos?
  No

[daffl]

Published as v8.1.1, thank you!