Always return NoopScope / NoopSpan for ScopeManager.Active / Tracer.ActiveSpan

[cwe1ss]

This brings opentracing/opentracing-java#263 to this repository.

Description from there:

In special circumstances, code may automatically generate spans, negating the need to always check if Tracer.activeSpan() is null. Under these circumstances, the Noop Tracer and ScopeManager will cause NPEs when they return null values when their active methods are accessed via the GlobalTracer. This
prevents code from running successfully when using Noop Tracers.

This minor change allows users to depend that their code will genuinely Noop under all use cases.

We have a separate NoopTests class in our repo so I had to adjust these tests. I also changed everything to AssertSame(Noop*.Instance, ...) instead of Assert.IsType<Noop*>(...) as this is a better check for our use case IMO.

As I changed these tests as well, I'll keep this PR open for a few days.

[cwe1ss]

Please file any objections until May 2nd - I'll merge afterwards if there's none.

[cwe1ss]

Found my own concerns 😄 and filed them here opentracing/specification#116. I'll put this on hold until that issue is resolved.