Don't allow assistive MathML to become part of the selection.  Resolves issue #1350.
Don't allow assistive MathML to become part of the selection.  Resolves issue #1350.

thomasjm/MathJax,thomasjm/MathJax,GerHobbelt/MathJax,jinntrance/mathjax,dpvc/MathJax,wikimedia/MathJax,thomasjm/MathJax,GerHobbelt/MathJax,thomasjm/MathJax,thomasjm/MathJax,jinntrance/mathjax