Fix TS types for creating and updating records with attachment fields

[okovpashko]

Fixes #277

I will appreciate suggestions and improvements regarding this change.

[okovpashko]

This is the only change I'm not sure about.

Without it, we get TS errors when passing fields with type eitherAttachmentReference or CreateAttachment to the create() or update() methods because the result of PartialFields has both fields' types and the indexed type from FieldSet:

interface MyFields extends FieldSet {
  name: string;
  files: Attachment[]
}

type PartialMyFields = PartialFields<MyFields>;

/*
The result type of PartialMyFields is
{
  [x: string]:
      | undefined
      | string
      | number
      | boolean
      | Collaborator
      | ReadonlyArray<Collaborator>
      | ReadonlyArray<string>
      | ReadonlyArray<Attachment | CreateAttachment | AttachmentReference>;
  name: string;
  files: (AttachmentReference | CreateAttachment)[]
}
*/

I will appreciate suggestions for a better solution.