[minor feature] Show undefined props that break schema better

[natew]

Case

let things = await Rx.Collection({ schema: { title: { type: 'string' } } })
things.insert({
   title: 'passesvalue',
   body: undefined
})

Issue

This will throw an error thats a bit confusing. In my case body doesn't exist on schema, notice it's undefined but in the error message you don't see that:

Because it doesn't show in the error the undefined it looks like the object should pass the schema.

Proposed solution would just be sure to log out the undefined value that causes it to fail.

[pubkey]

Hi @natew
There is no undefined-value in json. I now transform fields with undefined to null to ensure the field is not denied in the error-message.