ob-exp: Fix switches handling upon exporting

* lisp/ob-exp.el (org-babel-exp-code-template): Include switches in
  template.
(org-babel-exp-code): Provide %switches placeholder.
* testing/lisp/test-ob-exp.el (ob-export/export-src-block-with-switches):
  New test.
(ob-export/export-src-block-with-flags): Fix indentation.

This fixes dde6af3a6230b37aabfb4f75c2dee89433958375.  The confusion
came from the fact that "flags" placeholder had two meanings:
the :flags value and the block's switches (e.g., "-n").  This patch
separates these two meanings.

lisp/ob-exp.el

@@ -309,7 +309,7 @@ The function respects the value of the :exports header argument."
 	     (org-babel-exp-code info)))))
 
 (defcustom org-babel-exp-code-template
-  "#+BEGIN_SRC %lang%flags\n%body\n#+END_SRC"
+  "#+BEGIN_SRC %lang%switches%flags\n%body\n#+END_SRC"
   "Template used to export the body of code blocks.
 This template may be customized to include additional information
 such as the code block name, or the values of particular header
@@ -319,6 +319,7 @@ and the following %keys may be used.
  lang ------ the language of the code block
  name ------ the name of the code block
  body ------ the body of the code block
+ switches -- the switches associated to the code block
  flags ----- the flags passed to the code block
 
 In addition to the keys mentioned above, every header argument
@@ -341,8 +342,10 @@ replaced with its value."
    org-babel-exp-code-template
    `(("lang"  . ,(nth 0 info))
      ("body"  . ,(org-escape-code-in-string (nth 1 info)))
+     ("switches" . ,(let ((f (nth 3 info)))
+		      (and (org-string-nw-p f) (concat " " f))))
      ("flags" . ,(let ((f (assq :flags (nth 2 info))))
-		   (when f (concat " " (cdr f)))))
+		   (and f (concat " " (cdr f)))))
      ,@(mapcar (lambda (pair)
 		 (cons (substring (symbol-name (car pair)) 1)
 		       (format "%S" (cdr pair))))

testing/lisp/test-ob-exp.el

@@ -303,10 +303,21 @@ Here is one at the end of a line. =2=
       (org-export-execute-babel-code)
       (buffer-string)))))
 
+(ert-deftest ob-export/export-src-block-with-switches ()
+  "Test exporting a source block with switches."
+  (should
+   (string-match
+    "\\`#\\+BEGIN_SRC emacs-lisp -n -r$"
+    (org-test-with-temp-text
+	"#+BEGIN_SRC emacs-lisp -n -r\n\(+ 1 1)\n#+END_SRC"
+      (org-export-execute-babel-code)
+      (buffer-string)))))
+
 (ert-deftest ob-export/export-src-block-with-flags ()
   "Test exporting a source block with a flag."
   (should
-   (string-match "\\`#\\+BEGIN_SRC emacs-lisp -some-flag$"
+   (string-match
+    "\\`#\\+BEGIN_SRC emacs-lisp -some-flag$"
     (org-test-with-temp-text
 	"#+BEGIN_SRC emacs-lisp :flags -some-flag\n\(+ 1 1)\n#+END_SRC"
       (org-export-execute-babel-code)