test: fix mathml mime type detection.
test: fix mathml mime type detection.

It looks like the mime type for mml files has changed.

wavewave/lasem,janmarthedal/mathml2svg,wavewave/lasem,janmarthedal/mathml2svg,gjtorikian/lasem,wavewave/lasem,gjtorikian/lasem,gjtorikian/lasem,janmarthedal/mathml2svg,wavewave/lasem,janmarthedal/mathml2svg,gjtorikian/lasem