During the review process for adding opentracing factories, a bug was
introduced which caused Jaeger to initialize an invalid tracer. The
bug was due to use sending nil through as a kwarg when the Jaeger
initializer used a non-nil default value.

This is fairly insidious as, the tracer looks like a tracer, but, when
methods are invoked, it throws `NoMethodError` errors. To ensure that
this issue does not happen in future, the tests have been changed to
ensure that the tracer works as expected. This could avoid problems
in future when upgrading to newer versions of Jaeger.