JSExpr should consider serializing nothing as undefined

[clarkevans]

Currently, the JSON module serializes both nothing and missing to null. However, if you know your target language is Javascript, a more appropriate target would convert nothing to undefined.

In a similar light, NaN doesn't have to be converted to null, it could just remain NaN. On the other hand, Julia's Inf would need to be converted to Javascript's equivalent, Infinity.