Support private instance_variables_to_inspect (#70)

* Support private instance_variables_to_inspect in pp

Ruby supports calling instance_variables_to_inspect even when
it is defined as a private method (ruby/ruby#13555).

This change aligns pp with Ruby's behavior.

Author: hamajyotan

lib/pp.rb

@@ -388,7 +388,7 @@ def pretty_print_cycle(q)
     # This method should return an array of names of instance variables as symbols or strings as:
     # +[:@a, :@b]+.
     def pretty_print_instance_variables
-      ivars = respond_to?(:instance_variables_to_inspect) ? instance_variables_to_inspect : instance_variables
+      ivars = respond_to?(:instance_variables_to_inspect, true) ? instance_variables_to_inspect || instance_variables : instance_variables
       ivars.sort
     end
 

test/test_pp.rb

@@ -146,7 +146,9 @@ def a.pretty_print_instance_variables() [:@b] end
 
   def test_iv_hiding_via_ruby
     a = Object.new
-    def a.instance_variables_to_inspect() [:@b] end
+    a.singleton_class.class_eval do
+      private def instance_variables_to_inspect() [:@b] end
+    end
     a.instance_eval { @a = "aaa"; @b = "bbb" }
     assert_match(/\A#<Object:0x[\da-f]+ @b="bbb">\n\z/, PP.pp(a, ''.dup))
   end