[O] Babel should not work in the subtree marked as not exported

[zwz]

In my setup, there is 
(setq org-export-exclude-tags '("private" "exclude")

and In my test.org:

* test

** Not exported                :exclude:
   #+BEGIN_SRC ditaa :file test.png :cmdline -E
          +--------+---+---+---+---+---+---+---+  
+---+---+---+---+---+---+---+---+
      x   | 0 cRED | 0 | 0 | 0 | 0 | 0 | 0 | 0 |  | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 
1 |
          +--------+---+---+---+---+---+---+---+  
+---+---+---+---+---+---+---+---+
   #+END_SRC

** blah blah
   blah blah blah


When I try to export it to pdf, the test.png is still generated,
although it is not used for the pdf at all.
So I think the export procedure may be optimized for more efficiency.