fix(medusa): keep country_code when partially updating cart addresses (#2683)

**What**
- Fixes a bug where `country_code` was dropped when partially updating a shipping or billing address on a cart.

integration-tests/plugins/__tests__/medusa-plugin-sendgrid/__snapshots__/index.js.snap

@@ -2415,7 +2415,7 @@ Object {
       "address_2": null,
       "city": "ville la something",
       "company": null,
-      "country_code": null,
+      "country_code": "us",
       "created_at": Any<Date>,
       "customer_id": null,
       "deleted_at": null,
@@ -2476,4 +2476,4 @@ Object {
   "tracking_links": Array [],
   "tracking_number": "",
 }
-`;
+`;