The interface allows passing both Span and SpanContext as an argument
for creating References.
But the `instanceof` check won't always work in runtime, for example, for cases
when Span implementation does not extend the opentracing.Span class, but just
implements the interface.
The popular `jaeger-client-node` library does exactly that.

This produces hard-to-track errors.

I added additional checks to distinguish between Span and SpanContext instances.

Resolves: opentracing#161

Remove unnecessary fixtures, use simpler approach.

Co-authored-by: Yuri Shkuro <yurishkuro@users.noreply.github.com>