[代码修复](v2.6)：修复在新增/编辑菜单中切换外链菜单按钮时表单元素不再变化 (#107)

* fix v-show bug when switch is_external_link button in menu/index.vue

* More rigorous

src/views/system/menu/index.vue

@@ -59,7 +59,7 @@
           <el-input v-model="form.title" placeholder="按钮名称" style="width: 178px;" />
         </el-form-item>
         <el-form-item v-show="form.type.toString() !== '0'" label="权限标识" prop="permission">
-          <el-input v-model="form.permission" :disabled="form.iframe" placeholder="权限标识" style="width: 178px;" />
+          <el-input v-model="form.permission" :disabled="form.iframe.toString() === 'true'" placeholder="权限标识" style="width: 178px;" />
         </el-form-item>
         <el-form-item v-if="form.type.toString() !== '2'" label="路由地址" prop="path">
           <el-input v-model="form.path" placeholder="路由地址" style="width: 178px;" />
@@ -67,10 +67,10 @@
         <el-form-item label="菜单排序" prop="menuSort">
           <el-input-number v-model.number="form.menuSort" :min="0" :max="999" controls-position="right" style="width: 178px;" />
         </el-form-item>
-        <el-form-item v-show="!form.iframe && form.type.toString() === '1'" label="组件名称" prop="componentName">
+        <el-form-item v-show="form.iframe.toString() !== 'true' && form.type.toString() === '1'" label="组件名称" prop="componentName">
           <el-input v-model="form.componentName" style="width: 178px;" placeholder="匹配组件内Name字段" />
         </el-form-item>
-        <el-form-item v-show="!form.iframe && form.type.toString() === '1'" label="组件路径" prop="component">
+        <el-form-item v-show="form.iframe.toString() !== 'true' && form.type.toString() === '1'" label="组件路径" prop="component">
           <el-input v-model="form.component" style="width: 178px;" placeholder="组件路径" />
         </el-form-item>
         <el-form-item label="上级类目" prop="pid">