Description input in span/setStatus should be optional

Description is currently required:
```
>> tp = opentelemetry.sdk.trace.TracerProvider();
>> tr = getTracer(tp, 'ab');
>> sp = startSpan(tr, "abcdefghi");
>> sp.setStatus("ok")
Not enough input arguments.

Error in opentelemetry.trace.Span/setStatus (line 135)
            description = opentelemetry.common.mustBeScalarString(description);
```