[new feature] Tab support sticky (#382)

test/unit/components/tabs.vue

@@ -1,5 +1,5 @@
 <template>
-  <van-tabs :active="active" :duration="duration" @click="handleTabClick" @disabled="handleTabDisabledClick">
+  <van-tabs :active="active" :duration="duration" @click="handleTabClick" @disabled="handleTabDisabledClick" :sticky="sticky">
     <van-tab :title="firstTabTitle" :disabled="firstTabDisabled">内容一</van-tab>
     <van-tab title="选项二">内容二</van-tab>
     <van-tab title="选项三" disabled>内容三</van-tab>
@@ -24,7 +24,8 @@ export default {
     },
     firstTabDisabled: {
       type: Boolean
-    }
+    },
+    sticky: Boolean
   },
 
   data() {

test/unit/specs/tabs.spec.js

@@ -113,4 +113,20 @@ describe('Tabs', () => {
       done();
     });
   });
+
+  it('sticky', (done) => {
+    wrapper = mount(TabsTestComponent, {
+      attachToDocument: true,
+      propsData: {
+        sticky: true
+      }
+    });
+
+    wrapper.vm.sticky = false;
+
+    setTimeout(() => {
+      expect(wrapper.vm.$children[0].fixed).to.be.false;
+      done();
+    }, 30);
+  });
 });