Tracer.__init__() shouldn't share the default ScopeManager.

We want to be on the safe side: now by default we let
scope_manager=None, and upon checking it within __init__(),
create a new ScopeManager if needed, so instances of Tracer don't share it.

opentracing/tracer.py

@@ -39,8 +39,9 @@ class Tracer(object):
 
     _supported_formats = [Format.TEXT_MAP, Format.BINARY, Format.HTTP_HEADERS]
 
-    def __init__(self, scope_manager=ScopeManager()):
-        self._scope_manager = scope_manager
+    def __init__(self, scope_manager=None):
+        self._scope_manager = ScopeManager() if scope_manager is None \
+                else scope_manager
         self._noop_span_context = SpanContext()
         self._noop_span = Span(tracer=self, context=self._noop_span_context)
         self._noop_scope = Scope(self._scope_manager, self._noop_span)