Raise error when env value is not String
Raise error when env value is not String

I'd like to prevent these common mistakes.

- `FOO: #{foo}`
  - `#{foo}` must be quoted because `#` is parsed as comment
- `FOO_RATIO: 0.5`
  - Since all values were converted with `to_s`, it could contain errors
- `FOO: on`
  - Since `on` is a boolean keyword in YAML, `FOO` is set to `"true"`.

eagletmt/hako,eagletmt/hako,eagletmt/hako