text equation: unexpected whitespace when tag includes math mode

[pkra]

Found in tran8745

Minimal use case:

\begin{textequation}\tag{HR}\label{HR1}
    A
\end{textequation}
\begin{textequation}\tag{$HR$}\label{HR1}
B
\end{textequation}

becomes

  <disp-formula content-type="text" id="ltxid4">
    <p>A</p>
    <tag parens="yes">HR</tag>
  </disp-formula>
  <disp-formula content-type="text" id="ltxid5">
    <p>B</p>
    <tag parens="yes">
      <inline-formula content-type="math/tex">
        <tex-math>HR</tex-math>
      </inline-formula>
    </tag>
  </disp-formula>

The additional whitespace in the second tag element is unexpected. Downstream we end up with spaces after/before parentheses.

[pkra]

A solution might be via #231.