Add spec for defined? on constant with const_missing

Make sure that `defined?` returns nil even if `const_missing` is
implemented in the outer module.

language/defined_spec.rb

@@ -900,6 +900,10 @@
     defined?(DefinedSpecs::Undefined).should be_nil
   end
 
+  it "returns nil when the constant is not defined and the outer module implements .const_missing" do
+    defined?(DefinedSpecs::ModuleWithConstMissing::Undefined).should be_nil
+  end
+
   it "does not call .const_missing if the constant is not defined" do
     DefinedSpecs.should_not_receive(:const_missing)
     defined?(DefinedSpecs::UnknownChild).should be_nil

language/fixtures/defined.rb

@@ -285,6 +285,12 @@ def method_no_args
     end
   end
 
+  module ModuleWithConstMissing
+    def self.const_missing(const)
+      const
+    end
+  end
+
   class SuperWithIntermediateModules
     include IntermediateModule1
     include IntermediateModule2