Fix updates' tests nomenclature.
Add failing tests for refUpdate with ci === null.

[ci skip]
issue #12
Ignore non present context_ids in refUpdate.

Tests now pass.
Add tests for modifying context_ids from/to null.

[ci skip]
Issue #12
Only ignore changes to undefined contexts.

This applies only if both the current and old context_ids are undefined.
Therefore, only when there's absolutely no reference to update.

Tests now pass. This closes #12.
Bump version to 2.0.1