Docx reader: Fixes to block Normalization

These patches offer some fixes to the block normalization functions in yesterday's tab pull-request. They 

 1. fix two problems (`blockNormalize` didn't deal with Headers quite right, and left out DefinitionLists)

 2. simplify the definition of `blockNormalize`,

 3. add a comment explain why I'm sticking with a homemade string normalization function instead of using `normalize` from Text.Pandoc.Shared. (It more than doubles run time on tests on some large files.)

The heaeder error actually passed the tests. The issue is that the `toString` function in Tests.Helpers normalizes, so it makes it hard to test normalization. I'll be adding a different test soon to cover this.