Bug 50831: Support broken </br>

This is pretty common and easy to support.

Without selser, this will round-trip without the broken /. With selser, there
will not be any diff at all.

Also added a test. html2html round-tripping is currently broken for new
<p><br></p>, which is tracked in bug 51055.

Change-Id: I293b7e9fc5e90abaa755d0efa90c4d5bbe5753cb
Merge "Bug 50831: Support broken </br>"