Make BZ2File's fileobj support easier to use.

The fileobj argument was added during the 3.3 development cycle, so this change
does not break backward compatibility with 3.2.
diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index 8549677..6d73569 100644
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -1657,8 +1657,8 @@
         except ImportError:
             raise CompressionError("bz2 module is not available")
 
-        fileobj = bz2.BZ2File(filename=name if fileobj is None else None,
-                mode=mode, fileobj=fileobj, compresslevel=compresslevel)
+        fileobj = bz2.BZ2File(fileobj or name, mode,
+                              compresslevel=compresslevel)
 
         try:
             t = cls.taropen(name, mode, fileobj, **kwargs)