fix: allow clearing a to-one relationship with an explicit null id

[caiotarifa]

Closes #6

Problem

There was no way to clear a to-one relationship. JSON:API defines data: null for it, but the serializer could not produce that shape:

• field: null became an attribute (attributes.field = null).
• field: { type: 'documents', id: null } threw All included resources must have an ID.

Meanwhile field: [] already produced relationships.field.data = [] for the to-many case.

Change

A relationship object with an explicit id: null now serializes to relationships.<key>.data = null and is not collected into included. Because extractResource is shared, this works for nested resources too.

await api.update('article', {
  id: '1',
  author: { id: null }, // relationships.author.data = null
  tags: []              // relationships.tags.data = []
})

Behavior kept on purpose:

• An object with no id key still throws All included resources must have an ID., so a forgotten key stays a loud error instead of silently clearing a relationship.
• A bare field: null is still an attribute — the shape alone cannot tell an attribute from a relationship.

Tests

Three new cases in tests/serialize.test.ts: with type, without type, and on a nested resource. Full suite passes (64), plus typecheck, lint, and build.

Release

Version bumped to 2.2.0 with a CHANGELOG.md entry. Clearing relationships is documented in the README and on the docs site.

Not included

The issue also notes that a pre-built JSON:API document passed to request() is dropped when type is absent. Left out of this PR, as the author suggested tracking it separately.

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