handle boolean type values which not wrapped with quotes.

[pjc0247]

In standard json format, json can accept 'null', 'false' and 'true' values. but your implementation cannot parse boolean types which not wrapped with quotes("").

expected result

JSON_CLASS(Foo)
{
public:
  FIELD(bool, b);
}

{
  "b" : true
}

[jwvg0425]

fixed.