double.as_null_object always returns true for respond_to?

- Closes #16.

lib/rspec/mocks/mock.rb

@@ -34,6 +34,10 @@ def to_s
 
       alias_method :to_str, :to_s
 
+      def respond_to?(sym, incl_private=false)
+        __mock_proxy.respond_to?(sym, incl_private)
+      end
+
     private
 
       def method_missing(sym, *args, &block)

lib/rspec/mocks/proxy.rb

@@ -37,6 +37,10 @@ def initialize(object, name=nil, options={})
       def null_object?
         @null_object
       end
+
+      def respond_to?(sym, incl_private=false)
+        null_object? ? true : super
+      end
 
       # Tells the object to ignore any messages that aren't explicitly set as
       # stubs or message expectations.

spec/rspec/mocks/null_object_mock_spec.rb

@@ -35,6 +35,10 @@ module Mocks
         @mock.message(:expected_arg)
         @mock.message(:unexpected_arg)
       end
+
+      it "responds to everything" do
+        @mock.should respond_to(:any_message_it_gets)
+      end
     end
 
     describe "#null_object?" do