Handle serialization of odd float values

[mjgpy3]

Unfortunately System.Double.NaN, System.Double.PositiveInfinity and System.Double.NegativeInfinity cause invalid JSON to be generated. These seem like special cases that shouldn't happen often but they are not disallowed by the type system.

I was using FsCheck to generate JsonValue instances and I noticed an array containing NaN which causes JSON parsers to error.

This change attempts to mimic the decisions already made in JavaScript, e.g.

$ node
> JSON.stringify(NaN)
'null'
> JSON.stringify(Infinity)
'null'
> JSON.stringify(-Infinity)
'null'

[mjgpy3]

I just looked through the issues and it appears that this resolves #1259