Make sure explicit attributes added by \mmlToken are not removed. (mathjax/MathJax#2806)

[dpvc]

This PR modifies \mmlToken to make sure the attributes specified by it are not removed but the cleanAttributes filter, even if their values are the default values. This was causing the spacing for \bmod to be lost (and so it failed to have the proper space in super and subscripts).

Resolves mathjax/MathJax#2806.

[pkra]

I think this lets attributes bleed through to the output -- with this, I'm gettting underaccent="false" on g elements (from underset, I think).

[dpvc]

OK, thanks, I'll check into it.

[pkra]

Sorry for not being more conclusive -- I'm testing issue2828, issue2806, issue2766, issue2764 and SRE4 final all at once. Lots of tiny visual changes but nothing too bad so far.

[dpvc]

@pkra, no problem. We appreciate your reporting the issues.

[dpvc]

OK, I found it. It is in #763, where I used underaccent instead of accentunder. OOPS! Because underaccent is not a MathML attribute, it is treated as one that you have added yourself, and so retains it in the output. I'll make a fix for the attributes. Thanks for the report, which will save me some more public embarrassment.

[pkra]

Ah, sorry for misreporting -- I should've started by just updating from develop.

[dpvc]

No problem. I was pretty sure where the problem came from.

[zorkow]

lgtm.