babel: Added (:tangle . "no") to the default header arguments

* lisp/ob.el (org-babel-default-header-args): Added (:tangle . "no")
  to the default header arguments.  This is already the intended
  behavior, but it's better to be more explicit about such things.

lisp/ob.el

@@ -237,7 +237,7 @@ header arguments as well.")
 
 (defvar org-babel-default-header-args
   '((:session . "none") (:results . "replace") (:exports . "code")
-    (:cache . "no") (:noweb . "no") (:hlines . "no"))
+    (:cache . "no") (:noweb . "no") (:hlines . "no") (:tangle . "no"))
   "Default arguments to use when evaluating a source block.")
 
 (defvar org-babel-default-inline-header-args