Expander: the expanded spec is unusable to resolve circular $ref

[fredbi]

This is a nasty bug: so far I've failed to nail it.

When expanding schemas in a spec for further use (e.g. validating the spec, flattening it with the expand option),
circular $ref are left in a unusable state in the $ref cache, leading to jsonpointer errors (e.g. "xxx has no key yyy")
when attempting to resolve.

This is for example the root cause for go-swagger/go-swagger#2346.
In go-swagger, it is possible to work around this issue by reloading the spec as a fresh go-openapi/loads.Document,
but this is not ideal.
This workaround is used on 2 occasions in go-swagger:

• https://github.com/go-swagger/go-swagger/blob/55e764c3e84b9d3779835bb0513d7fd292b0a303/generator/spec.go#L54
• fix(flatten): fixed code generation with circular $ref AND expand option go-swagger/go-swagger#3020 (use workaround to fix the above-mentioned issue)

[fredbi]

The issue has been traced as originating from validate.Spec (or validate.AgainstSchema) which mutates the input spec or schema by proceeding to expand. Since v0.26.4, this pollution of the input no longer occurs and the pure outcome of expand failed to reproduce the issue in other conditions. Closed by go-openapi/validate#291