Eliminate added x-scope attributes

[wittlesouth]

I'm working on some code that imports an OpenAPI spec, modifies it, and outputs a new one. I'd prefer not to have the spec modified by the validator; the addition of the x-scope attributes is something I'd prefer to avoid.

I'm working around this for now by making a deep copy of the spec and passing that to validate_spec, so consider this a suggestion rather than a serious issue.