Bug fix in serialiation of <div attr="">foo</div> style tags

* This would RT to <div attr>foo</div> -- and is the cause of
  a number of noisy diffs in rt-testing.

* This patch fixes this to serialize to <div attr="">foo</div>
  always.

* Fixes a couple wt2wt tests, and 1 html2wt test, and a few
  selser tests as well.

* This patch also normalizes <font color>foo</font> to
  <font color="">foo</font> which is the new failing wt2wt test
  (and associated selser failures because selser is more accurate).

Change-Id: Iaa5f2d586e01ba60fc9ff1325c3c8bda9cab1b66