Bugfix: do not generate invalid JSON when float value is +Inf, -Inf or NaN

[dmitrybarsukov]

This PR solves issue #134 - Invalid JSON generated for NaN and Inf.

Context:
The following code generates invalid JSON, which cannot be unmarshalled either via easyjson or builtin go encoding/json package.

package main

import (
	"fmt"
	"math"
)

//go:generate easyjson $GOFILE

//easyjson:json
type Struct struct {
	Float1 float64
	Float2 float64
	Float3 float64
}

func main() {
	s := Struct{
		Float1: math.NaN(),
		Float2: math.Inf(1),
		Float3: math.Inf(-1),
	}
	b, err := s.MarshalJSON()
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Println(string(b))
	var s2 Struct

	if err := s2.UnmarshalJSON(b); err != nil {
		fmt.Println(err)
		return
	}
	fmt.Println(s2)
}

Output:

{"Float1":NaN,"Float2":+Inf,"Float3":-Inf}
parse error: syntax error near offset 10 of 'NaN,"Float...'

Suggested solution:

In jwriter/writer.go: check floats for NaN, +Inf, -Inf values before passing them into strconv.AppendFloat, and write w.Error = "json: unsupported value: ..." if it is so