Add spec for defined? on constant with const_missing

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

language/defined_spec.rb

@@ -900,6 +900,14 @@
     defined?(DefinedSpecs::Undefined).should be_nil
   end
 
+  it "returns nil when the constant is not defined and the parent implements const_missing" do
+    def DefinedSpecs.const_missing(const)
+      const
+    end
+
+    defined?(DefinedSpecs::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