Use private_method_defined? instead of respond_to?

`Module.respond_to?(:ruby2_keywords, true)` does NOT check if
`Module#ruby2_keywords` is available. It worked well because there is
toplevel `ruby2_keywords` method, but using `private_method_defined?` is
better, I think.

Also, this fixes a syntactic error.

Author: mame

lib/ruby2_keywords.rb

@@ -1,5 +1,5 @@
 class Module
-  unless respond_to?(:ruby2_keywords, true)
+  unless private_method_defined?(:ruby2_keywords, true)
     private
     def ruby2_keywords(name, *)
       # nil
@@ -23,12 +23,9 @@ def ruby2_keywords
 end
 
 class Hash
-  unless respond_to?(:ruby2_keywords_hash?)
-    begin
-      $VERBOSE, verbose = nil, $VERBOSE
-      proc {|*a, **h| h}.call(
-    ensure
-      $VERBOSE = verbose
+  unless method_defined?(:ruby2_keywords_hash?)
+    def ruby2_keywords_hash?
+      false
     end
   end
 end