Make NullTracer pass spanOptions to constructor (#202)

Pass spanOptions through to the newly created Span instead of always using
default values.

The Span constructor does additional work if there's no options passed in.
Instrumented code that uses the NullTracer (as a result of not sampling or
other reasons) can avoid this work by passing the options that are already
set.

This change also makes the use of the stackTrace option more consistent. It
preserves the passed-in stackTrace, which may be pre-filtered or an empty array.
The current behavior leads to the unexpected outcome that the non-sampled
requests were taking deubg_backtraces when the instrumented route was passing
in its own value.

src/Trace/Tracer/NullTracer.php

@@ -52,7 +52,7 @@ public function inSpan(array $spanOptions, callable $callable, array $arguments
      */
     public function startSpan(array $spanOptions)
     {
-        return new Span();
+        return new Span($spanOptions);
     }
 
     /**