Fix Panic on Introspection of Schema using ToJSON

Introspection via ToJSON works differently when used via the
 `Schema.ToJSON` function than when making an introspection query from a
 client such as GraphiQL. In the later case, introspection uses the
 Schema's registered resolver, while in the former case, a
 `resolvable.Schema` is faked

Without the `Meta` being set on this `resolvable.Schema` (following the
 recent changes to address race conditions around the Meta schema), this
 resulted in a panic when using this form of schema introspection.