Add option to update instead of replace document during save

[mikestead]

From what I could determine, any time you call save() on an existing document, the update overwrites the document in the DB.

If you just want to update a single attribute in the document you first need to load the entire document into memory and then send the entire document back over the wire with the updates.

This change provides the option to update specific attributes of an existing document.

Example

const users = await User.where('username', 'user-a').include('height').find()
const user = users[0]
if (user.get('height') !== newHeight) {
  user.set('height', newHeight)
  await user.save({ merge: true })
}

I'm not sure merge is the best term? overwrite: false might be better but then we need to ensure it's truthy everywhere as default.

Ref https://docs.mongodb.org/manual/reference/operator/update/set

[coveralls]

Coverage increased (+0.04%) to 93.868% when pulling 49ecfd6 on mikestead:feature/update-merge into 72433a7 on vdemedes:master.

[vadimdemedes]

What if we make this a default behavior? Send only this.changed attributes and use $set: true? Can you think of any downsides?

[mikestead]

Sounds good, I think it's more intuitive to do a $set instead of a replace by default.

I guess the main issue is that it's a breaking change so should require another major version bump?

[vadimdemedes]

Don't think that it's a breaking change, as API doesn't change and the result of .save() should be the same too.

[mikestead]

The original example is a little simplified but given that document had more than just the property height, without the change you would have a document left with just height in it. Anyone depending on that behaviour would be broken. I'm not sure how common that would be, but is a possibility.