Adapt to changed mongoose behaviour

[vinerich]

Summary (Fixes #71)

• Bumps mongoose to newest version
• Adapts to mongoose changes

Explanation

Somewhere along the versions 5.9 to 5.10 several changes where introduced:

• Plugin hook functions must (!) call next(), simply calling return while leave a non-returning promise.
• When upserting a document the query result changed from { nModified: 1, ... } to { nModified: 0, upserted: [docs], ... }

So I changed the nModified check to include the new upserted property and am calling next() to return.

Additional

In #72 I tested my fix on different mongoose versions and it passed on both. I also tested with more versions on my machine.
However this tests are not included into CI. I'm not sure if running 30 different version combinations is a good way to go.

I would propose to include Node v14 in the test, but only run with the newest mongoose versions. Your opinion @codepunkt?

[vinerich]

@codepunkt Maybe consider merging this PR. Last bump, I promise 😄