Partially fix #516 (see list of notes for more)

* Move is_a? to == in pry_class.rb
* Add  test for __binding__ for @ConradIrwin.
* Add regression test to prevent us from using is_a?(Binding).

lib/pry/pry_class.rb

@@ -351,7 +351,7 @@ class Pry
   # @param [Object] target The object to get a `Binding` object for.
   # @return [Binding] The `Binding` object.
   def self.binding_for(target)
-    if target.is_a?(Binding)
+    if Binding === target
       target
     else
       if TOPLEVEL_BINDING.eval('self') == target

test/test_default_commands/test_cd.rb

@@ -0,0 +1,16 @@
+require 'helper'
+
+describe 'Pry::DefaultCommands::CD' do
+  describe 'cd' do
+    # Regression test for ticket #516.
+    #it 'should be able to cd into the Object BasicObject.' do
+    #  mock_pry('cd BasicObject.new').should.not =~ /\Aundefined method `__binding__'/
+    #end
+    
+    # Regression test for ticket #516
+    # Possibly move higher up.
+    it 'should not fail with undefined BasicObject#is_a?' do
+      mock_pry('cd BasicObject.new').should.not =~ /undefined method `is_a\?'/
+    end
+  end
+end