In MS Word 2015 (and possibly others), leading and trailing spaces are
ignored in text runs. This means that calling TextRun with
leading/trailing space would result in a document that didn't include
those spaces. The fix here (per http://officeopenxml.com/WPtext.php)
is to include an extra attribute on the "w:t" element that forces word
to recognize those spaces.
Instead of forcing table cells to only have a single paragraph as
their content, we now check whether they end in a paragraph (and
insert one if necessary) during #prepForXml
