Allow `cid` to be a string

I was loading the configuration for `Expedia.cid` directly from a `Rails.configuration` value like this:

```ruby
Expedia.cid = Rails.configuration.expedia_cid
```

When doing this, the value supplied will always be a String. This was causing authentication to fail with Expedia as the production server was being used instead of the development server. I think this is a very common use case. This commit fixes potential head scratching.