Prefer Nan to NaN

[Rust's RFC 0430 about naming
conventions](https://github.com/rust-lang/rfcs/blob/master/text/0430-finalizing-naming-conventions.md#fine-points)
says:

> In `CamelCase`, acronyms count as one word: use `Uuid` rather than `UUID`.

Standard library follows this rule and uses `Nan` (see
`std::num::FpCategory::Nan`), so this crate had better use `Nan`.

Author: Takuma YOSHIOKA