Expose failure to normalize whitespaces

Run with:

    python -m doctest normalize.py

I found this while trying the CommonMark test suite with `pandoc`:

    ./test/spec_tests.py --program pandoc

Also I recommend that we add a bunch more unit doctests for that function similarly to: https://github.com/karlcow/markdown-testsuite/blob/639cd234d71ca81956b61ff7876f37c3cdc5c043/run-tests.py#L75 because it is very tricky to implement.