Test the result of getAttribute() after setAttribute(null).

The intended behaviour has become clear by now.
Reinstate tests for leading whitespace in numeric types.

The relevant bugs have been closed long ago.
Add a test for Node#nodeValue on a DocumentType.
Add some Document#getElementsByTagNameNS tests that should return empty lists.
Merge pull request #1341 from Ms2ger/reflection

Enable reflection tests that are no longer controversial.
Merge pull request #1345 from Ms2ger/doctype-nodeValue

Add a test for Node#nodeValue on a DocumentType.
Merge pull request #1346 from Ms2ger/getElementsByTagNameNS-empty

Add some Document#getElementsByTagNameNS tests that should return empty lists.