Default answer defaults to nil.

As @trans pointed out: not not all questions are necessarily "Yn".
And if default answer is nil, one can do

```ruby
if answer = ask("Some question?")
  # do something with answer
else
  # no answer
end
```

Author: weakish

lib/clik/ask.rb

@@ -6,7 +6,7 @@ module Kernel
   # via the console. A prompt will be sent to $stdout,
   # if given, and the input taken from $stdin...
   #
-  #   ask "Are you happy? [Yn]" "Y"
+  #   ask "Are you happy? [Yn]", "Y"
   #
   # On the command line one would see...
   #
@@ -19,7 +19,7 @@ module Kernel
   # The ask method would return "Y".
   #
   # Returns [String]
-  def ask(prompt=nil, default_answer=Y)
+  def ask(prompt=nil, default_answer=nil)
     $stdout << "#{prompt}"
     $stdout.flush
     ans = $stdin.gets.chomp!
@@ -29,6 +29,5 @@ def ask(prompt=nil, default_answer=Y)
       ans
     end
   end
-
 end