Fix: Allow <div /> as valid Math output

Some simple math tags such as the rendering result of
$\mathcal{ABC}$
are converted to an HTML-div tag by LaTeXML if inline-mode
is used. This change
* makes the $wgMathDefaultLaTeXMLSetting easier to read
* improves the XML validation of the LaTeXML output.

Change-Id: I807dbfa3a45930deec8eb5615133be9a39f3f101