Card: add test

packages/card/src/main.vue

 <template>
   <div class="el-card">
-    <div class="el-card__header" v-if="$slots.header">
+    <div class="el-card__header" v-if="$slots.header || header">
       <slot name="header">{{ header }}</slot>
     </div>
     <div class="el-card__body" :style="bodyStyle">

test/unit/specs/card.spec.js

+import { createVue, createTest } from '../util';
+import Card from 'packages/card';
+
+describe('Card', () => {
+  it('slot:header', () => {
+    const vm = createVue(`
+      <el-card>
+        <header slot="header">二师兄叫我埋梗 啦啦啦</header>
+      </el-card>
+    `);
+
+    expect(vm.$el.querySelector('.el-card__header')).to.property('textContent').to.include('二师兄叫我埋梗 啦啦啦');
+  });
+
+  it('header', () => {
+    const vm = createTest(Card, {
+      header: '好烦'
+    });
+
+    expect(vm.$el.querySelector('.el-card__header')).to.property('textContent').to.include('好烦');
+  });
+
+  it('bodyStyle', () => {
+    const vm = createTest(Card, {
+      bodyStyle: { padding: '10px' }
+    });
+
+    expect(vm.$el.querySelector('.el-card__body').style.padding).to.equal('10px');
+  });
+});