Define Kernel#instance_variables_to_inspect

[Bug #21718]

Otherwise objects that don't define it, but define a fairly liberal
`method_missing` method will run into errors that are hard to understand:

```ruby
class Foo
  def method_missing(name, ...)
    name
  end
end

p Foo.new.inspect
```

```
'Kernel#inspect': wrong argument type Symbol (expected Array) (TypeError)
	from ../test.rb:7:in '<main>'
```

1 files changed, 29 insertions, 0 deletions

diff --git a/spec/ruby/core/kernel/inspect_spec.rb b/spec/ruby/core/kernel/inspect_spec.rb
index 6ecf1e1c8c..1fa66cab98 100644
--- a/spec/ruby/core/kernel/inspect_spec.rb
+++ b/spec/ruby/core/kernel/inspect_spec.rb
@@ -57,5 +57,34 @@ describe "Kernel#inspect" do
       inspected = obj.inspect.sub(/^#<Object:0x[0-9a-f]+/, '#<Object:0x00')
       inspected.should == "#<Object:0x00>"
     end
+
+    it "displays all instance variables if #instance_variables_to_inspect returns nil" do
+      obj = Object.new
+      obj.instance_eval do
+        @host = "localhost"
+        @user = "root"
+        @password = "hunter2"
+      end
+      obj.singleton_class.class_eval do
+        private def instance_variables_to_inspect = nil
+      end
+
+      inspected = obj.inspect.sub(/^#<Object:0x[0-9a-f]+/, '#<Object:0x00')
+      inspected.should ==  %{#<Object:0x00 @host="localhost", @user="root", @password="hunter2">}
+    end
+
+    it "raises an error if #instance_variables_to_inspect returns an invalid value" do
+      obj = Object.new
+      obj.instance_eval do
+        @host = "localhost"
+        @user = "root"
+        @password = "hunter2"
+      end
+      obj.singleton_class.class_eval do
+        private def instance_variables_to_inspect = {}
+      end
+
+      ->{ obj.inspect }.should raise_error(TypeError, "Expected #instance_variables_to_inspect to return an Array or nil, but it returned Hash")
+    end
   end
 end