check_bundle allows type String, defaults config to {}

- when using type "json:nad", a symbol does not make sense. Since it's
  being converted to a string in the API call, why not just allow String
  as a type?
- default config to {} if not present. For checks that require a config,
  this gives more useful error messages about invalid configs instead of
  about invalid JSON.
Merge pull request #2 from wanelo-chef/check_bundle_defaults

check_bundle allows type String, defaults config to {}