Fix JsonParser.isNaN() to be stable, only indicate explicit NaN values; not ones due to coercion

[cowtowncoder]

First part of fixes related to problems with number Overflow/Underflow for Double leading to problems with BigDecimal access. Second part of solution will be #1149.

[cowtowncoder]

@pjfanning I think that this first part should help a bit: we'll simply keep track of input that is NaN and not consider possible Overflow/Underflow-induced cases actual NaNs (since they would be decodable into BigDecimal).
This allows some changes, but more importantly if this goes cleanly, can implement #1149.

/cc @JooHyukKim