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)