Allow _id as a primary key

[will118]

I have implemented #681

If there is an edge case or some behaviour not covered by my tests, let me know and I will add the test.

Apologies for the delay. I am committed to getting this merged so feedback is welcome.

Tests

RxCollection

• Inserting a document with _id set on a schema with _id: primary
• Inserting a document with _id set on a schema with somethingElse: primary
• Inserting a document with _id set on a schema with no primary defined (implicit _id)

RxSchema

• Creating a schema with _id as primary
• Creating a schema with _id not as primary

[pubkey]

Looks good.
One edge-case: What happens if an primary-field e.g. passportId is set, and I insert an object which contains _id and passportId? I think we should throw an error if this happens.

[will118]

@pubkey Great!

Does this logic not cover it:
https://github.com/pubkey/rxdb/pull/824/files#diff-ee35040c15aa81d65903334acff165c9R371

and this test:
https://github.com/pubkey/rxdb/pull/824/files#diff-162336d85de73bd515f48480759e07cdR375

I just tried writing a test for RxCollection where the schema has passportId (primary: true) and _id but of course this is rejected by SchemaCheck!

So I don't think it's possible to write any other tests for this case, let me know if you have any ideas.

[pubkey]

Yes you are right. I merged this. Thank you for the great work.