indent block boundaries when body is no-indent

Thanks to Seb and Nick for pointing this out.

* lisp/org-exp-blocks.el (org-export-blocks-preprocess): Even when the
  body of a block is not indented the boundary markers should be
  indented to their original positions so things like list indentation
  still work.

lisp/org-exp-blocks.el

@@ -211,7 +211,13 @@ which defaults to the value of `org-export-blocks-witheld'."
 		  (when replacement
 		    (delete-region match-start match-end)
 		    (goto-char match-start) (insert replacement)
-		    (unless preserve-indent
+		    (if preserve-indent
+			;; indent only the code block markers
+			(save-excursion
+			  (indent-line-to indentation) ; indent end_block
+			  (goto-char match-start)
+			  (indent-line-to indentation))	; indent begin_block
+		      ;; indent everything
 		      (indent-code-rigidly match-start (point) indentation)))))
 	      ;; cleanup markers
 	      (set-marker match-start nil)