Fix incorrect assignments content-mathml extension, and don't modify original document.  Resolves issue #1575.
Fix incorrect assignments content-mathml extension, and don't modify original document.  Resolves issue #1575.

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