Add specs for Memory#close

Author: jvazquez-r7

spec/lib/rex/image_source/memory_spec.rb

@@ -96,7 +96,9 @@
   end
 
   describe "#close" do
-
+    it "returns nil" do
+      expect(subject.close).to be_nil
+    end
   end
 
   describe "#index" do