ob-core: Allow never-export as :eval value

* lisp/ob-core.el (org-babel-common-header-args-w-values): Allow
  "never-export" as :eval value.

lisp/ob-core.el

@@ -453,7 +453,8 @@ then run `org-babel-switch-to-session'."
     (colnames	. ((nil no yes)))
     (comments	. ((no link yes org both noweb)))
     (dir	. :any)
-    (eval	. ((yes no no-export strip-export eval never query)))
+    (eval	. ((yes no no-export strip-export never-export eval never
+			query)))
     (exports	. ((code results both none)))
     (epilogue   . :any)
     (file	. :any)