REST API: Make JSON Schema enum more robust for non-string types

[TimothyBlynJacobs]

The enum JSON schema keyword allows for enforcing that the provided value is one of the listed values. This can be used with any type of value. Our implementation, however, doesn't apply the same type coercion when doing regular validation.

In other words, for an enum of [ 0, 1 ] I should be able to pass "1".

To do this, I think we might want to move the enum validation to the end of rest_validate_value_from_schema so that type validation has already been applied, and then either call rest_sanitize_value_from_schema before passing the value to the enum check, or just do the type casting.

We'll also need to pay special care to arrays/objects applying rest_stabilize_value and making sure that the type safe in_array() check has the correct JSON semantics.