fix #55 return the right type on json.num* call

[gkorland]

No description provided.

[gavrie]

OK, so I tried a bunch of different approaches, and finally settled on this one which isn't too different from the original code. It's the most concise with the least duplication. The most likely contender was based on traits, but it turned out much too verbose.

Please see my latest commits, and let's merge if you're happy with it.

[gkorland]

why do you need this (num1,num2)?

            let num_res = match (curr_value.as_i64(), in_value.as_i64()) {
                (Some(num1), Some(num2)) => op_i64(num1, num2).into(),
                _ => {
                    let num1 = curr_value.as_f64().unwrap();
                    let num2 = in_value.as_f64().unwrap();
                    Number::from_f64(op_f64(num1, num2)).unwrap()
                }
            };

Seems cleaner not to overload the num1,num2 with two types