Leave the housekeeping to the NSHashTable itself, which should purge
null-references automatically (not necessarily right away)
Also update tests accordingly: Use `allObjects.count` instead of simply
`count` on the NSHashTable, since `allObjects.count` returns non-null
references.
This should fix#36
The data property is longer exposed as a mutable string. Instead all
manipulations must be performed via the provided API in order to ensure
correct updates for the Document’s attached ranges.
See:
https://dom.spec.whatwg.org/#characterdata