MathMLinHTML: Fix deprecation warning

The use of `nodeValue` on `Attr` objects causes a deprecation warning in
Firefox since version 7.  This patch causes calls to `value` instead of
`nodeValue` for all `Attr` objects.

This closes #362.
Merge pull request #1257 from tarleb/mathmlinhtml-deprecation-warning

MathMLinHTML: Fix deprecation warning