[Math Processing Error] with mathvariant

Created by: fred-wang

The mathvariant attribute values for arabic scripts as well as unknow values can cause a [Math Processing Error].

The MathJax testsuite contains tests here:

MathMLToDisplay/Characters/Variants/

They are taken from the MathML test suite, but don't test the attributes mentioned above. Here is a testcase:

<math><mi mathvariant="inital">x</mi></math>
<math><mi mathvariant="tailed">x</mi></math>
<math><mi mathvariant="looped">x</mi></math>
<math><mi mathvariant="stretched">x</mi></math>
<math><mi mathvariant="unknown">x</mi></math>