Update status code key type definitions to be numbers

[dmmulroy]

Currently, the status code keys on the HttpStatus interface are typed incorrectly as strings.

For example:

interface HttpStatus {
  readonly '100': string;
}

When it should be

interface HttpStatus {
  readonly 100: string;
}

and you can see this to be true by looking at the real exported object: https://github.com/adaltas/node-http-status/blob/master/lib/index.js#L49