Original Title: dividing by zero makes field disappear

ExpressionDivide returns Undefined on divide by zero, which causes the field to be dropped. Might be more appropriate to return +/- Infinity or NaN.

c = db.c;
c.drop();

c.save( { a:NumberLong(10) } );
printjson( c.aggregate( { $project:{ z:{ $divide:[ '$a', 0 ] } } } ) );

Observed behavior: A field is removed on attempt to set its value to the output of a division by zero.
Expected behavior: The field is set to an appropriate value ( see above ) rather than removed.

The same occurs when attempting to mod by zero:

c.save( { a:NumberLong(10) } );
printjson( c.aggregate( { $project:{ z:{ $mod:[ '$a', 0 ] } } } ) );