Convert multi-word underscore keys to camel case

Currently, values that come back from calls like `https://www.googleapis.com/oauth2/v2/userinfo` or `https://www.googleapis.com/oauth2/v2/tokeninfo` don't quite work properly because the API returns some keys in snake_case, and the properties are defined on the corresponding models in cameCase. This means Google_Model->mapTypes doesn't map these keys and the getters on the corresponding model always return null.

This PR checks for the existence of the key as a camelCase property when running mapTypes, and assigns the value to that key if it exists.