Numbers are always be parsed to Value::F64

[zonyitoo]

{
  # specify rate in requests/second (because comments are helpful!)
  rate: 1000
}

This is the example taken from README. And we have to use the value of rate by as_f64 like this:

let rate = sample.get("rate").unwrap().as_f64().unwrap();

1000 should be Value::U64 or at least Value::I64.

Maybe we can represent number values as Value::Number like serde-json does?