Fix JSON serialization of Stripe objects

[ob-stripe]

r? @remi-stripe
cc @stripe/api-libraries

• Adds ToString and ToJson methods to StripeEntity
• Exclude StripeResponse from JSON serialization
• Fix serialization of expandable attributes
• Don't serialize deleted attribute when it's null
• Add a test for serialization of expandable fields

I realize that the new getters for expandable fields add a lot of boilerplate. In the future, I'll probably replace the object declarations with an ExpandableField<> generic class and a custom serializer/deserializer, similar to what we do in Java, but for now this does the job.

[ob-stripe]

Aw, Windows outputs CRLF line endings. Will fix.

edit: fixed.

[ob-stripe]

Added a wholesome test, ptal @remi-stripe