fix default value in interactiveshell.ask_yes_no

was True, should be None

closes  #1026

IPython/core/interactiveshell.py

@@ -2616,7 +2616,7 b' class InteractiveShell(SingletonConfigurable, Magic):'
         """Write a string to the default error output"""
         io.stderr.write(data)
 
-    def ask_yes_no(self,prompt,default=True):
+    def ask_yes_no(self, prompt, default=None):
         if self.quiet:
             return True
         return ask_yes_no(prompt,default)

IPython/core/magic.py

@@ -1058,7 +1058,8 b' Currently the magic system has the following functions:\\n"""'
             ans = True
         else:
             ans = self.shell.ask_yes_no(
-                "Once deleted, variables cannot be recovered. Proceed (y/[n])? ")
+                "Once deleted, variables cannot be recovered. Proceed (y/[n])? ",
+                default='n')
         if not ans:
             print 'Nothing done.'
             return