* Markdown writer now includes a blank line at the end
of the row in a single-row multiline table, to prevent it from being
interpreted as a simple table. Closes#4578.
* Markdown reader does a better job computing the relative width of
the last column in a multiline table, so we can round-trip tables
without constantly shrinking the last column.
This was failing because of a small discrepancy in markdown
table header line lengths on appveyor.
It's a minor issue, I can't see what is causing it, and
it's irrelevant to the issue this is testing, so we'll
just write native for this test.
