Docx Reader: All headers get auto id.

Previously, only those with an anchor got an auto id. Now, all do, which
puts it in line with pandoc's markdown extension.
Update docx tests to reflect new expected behavior

Now doing auto ids for all headers, so tests should reflect that.
Merge pull request #1379 from jkr/more_auto_id

Docx Reader: Extend auto id behavior to all headers