Get rid of block-less Proc.new

* lib/optparse.rb (OptionParser::Switch#initialize): get rid of
  `Proc.new` without a block, which may be deprecated in the future.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66069 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/optparse.rb

@@ -534,8 +534,9 @@ class OptionParser
 
     def initialize(pattern = nil, conv = nil,
                    short = nil, long = nil, arg = nil,
-                   desc = ([] if short or long), block = Proc.new)
+                   desc = ([] if short or long), block = nil, &_block)
       raise if Array === pattern
+      block ||= _block
       @pattern, @conv, @short, @long, @arg, @desc, @block =
         pattern, conv, short, long, arg, desc, block
     end