Update TeXAtom to save the texClass in the property list, so it appears in MathML output. (mathjax/MathJax#2585)

[dpvc]

This PR adjusts the TeXAtom node to save the node's TeX class in the property list so that it will be included in the data-mjx-texclass attribute when serialized MathML is generated, even if the TeX class is just the default ORD class. This allows the serialized MathML to preserve the TeXAtom even when the property was not set explicitly.

For example, f{\left(x\right)} currently produces

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mi>f</mi>
  <mrow>
    <mrow data-mjx-texclass="INNER">
      <mo data-mjx-texclass="OPEN">(</mo>
      <mi>x</mi>
      <mo data-mjx-texclass="CLOSE">)</mo>
    </mrow>
  </mrow>
</math>

and since the outer <mrow> does not include a TeX class, the spacing is that of INNER when this MathML is read into MathJax (but the spacing is ORD when the TeX input is processed).

This PR causes the serialized MathML to be

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mi>f</mi>
  <mrow data-mjx-texclass="ORD">
    <mrow data-mjx-texclass="INNER">
      <mo data-mjx-texclass="OPEN">(</mo>
      <mi>x</mi>
      <mo data-mjx-texclass="CLOSE">)</mo>
    </mrow>
  </mrow>
</math>

which gives the same output as the TeX does.

Note that this PR probably will require changes to the test suite, since braced groups will now show data-mix-texclass="ORD" explicitly in situations where it didn't before. (E.g., x^{-4}).

Resolves issue mathjax/MathJax#2585.

[zorkow]

lgtm.

[dpvc]

I want to do some additional checking before merging this one. There may be an issue with the class changing after the TeXAtom is created.

[dpvc]

OK, I've checked it out, and I think it is OK. Some of the values may not be strictly necessary, but it does look like they are all getting included in the output now.