Fixed common tags leaking state between spans (#9)

* fix bug where state is leaked between spans when using common tags feature

* fix linting issue

Author: rhilfers

haystack/tracer.py

@@ -105,7 +105,8 @@ def start_span(self,
 
         # Set common tags
         if self._common_tags:
-            tags = {**self._common_tags, **tags} if tags else self._common_tags
+            tags = {**self._common_tags, **tags} if tags else \
+                self._common_tags.copy()
 
         return Span(self,
                     operation_name=operation_name,