🐛 Fixed unnecessary "unsaved changes" modal when using HR cards

no issue

- the `hr` mobiledoc card does not specify an `absoluteToRelative` or `relativeToAbsolute` transformer function so falls back to the default transformer
- the default transformer function's arguments were not correct which meant that the UrlUtils object was replacing the card's typical empty-object payload
- the card's payload changing when saving mobiledoc was triggering the editor's unsaved changes warning because the API response no longer matched what was in the editor

core/server/lib/mobiledoc/create-card.js

@@ -1,7 +1,7 @@
 let urlUtils;
 
 module.exports = function createCard(card) {
-    const defaultTransformer = function (payload) {
+    const defaultTransformer = function (urlUtils, payload) {
         return payload;
     };
 

core/test/unit/lib/mobiledoc/cards/hr_spec.js

@@ -13,4 +13,16 @@ describe('HR card', function () {
 
         serializer.serialize(card.render(opts)).should.match('<hr>');
     });
+
+    it('transforms urls absolute to relative', function () {
+        let payload = {};
+        const transformed = card.absoluteToRelative(payload, {});
+        transformed.should.deepEqual({});
+    });
+
+    it('transforms urls relative to absolute', function () {
+        let payload = {};
+        const transformed = card.relativeToAbsolute(payload, {});
+        transformed.should.deepEqual({});
+    });
 });