add spec for Cache validating JSON contents

spec/rscons/cache_spec.rb

@@ -79,5 +79,12 @@ module Rscons
         expect(build_from({}).send(:calculate_checksum, "fname")).to eq "the_checksum"
       end
     end
+
+    describe "#validate_json_object" do
+      it "raises an error when an unexpected object is passed" do
+        cache = build_from({})
+        expect {cache.__send__(:validate_json_object, :symbol)}.to raise_error /Unexpected cache value/
+      end
+    end
   end
 end