feat(Image): add default slot (#6613)

src/image/test/__snapshots__/index.spec.js.snap

@@ -1,5 +1,12 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`default slot 1`] = `
+<div class="van-image"><img src="https://img.yzcdn.cn/vant/cat.jpeg" class="van-image__img">
+  <div class="van-image__loading"><i class="van-icon van-icon-photo-o van-image__loading-icon">
+      <!----></i></div>Custom Default
+</div>
+`;
+
 exports[`error-icon prop 1`] = `
 <div class="van-image">
   <div class="van-image__error"><i class="van-icon van-icon-error van-image__error-icon">

src/image/test/index.spec.js

@@ -157,3 +157,16 @@ test('radius prop', () => {
 
   expect(wrapper).toMatchSnapshot();
 });
+
+test('default slot', () => {
+  const wrapper = mount(VanImage, {
+    propsData: {
+      src: 'https://img.yzcdn.cn/vant/cat.jpeg',
+    },
+    scopedSlots: {
+      default: () => 'Custom Default',
+    },
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});