fix json(:symbol) returns "{}"

When i do ` json(:a) `, it will return `"{}"`. i think it should return `"a"`. So i fix it.

And i change this line https://github.com/yfractal/JSON.jl/blob/master/src/JSON.jl#L48 to `JSON.print(io, key)`

Hope i did right and please review this, thanks :)