iOS 11 tableView for header in xib !

## Summary

使用 xib 创建 tableView 的 header 的时候出现缺少20的距离

## How would you classify this issue?

Choose a problem area with iOS SDK：

* [ ] Foundation
* [x] UIKit
* [ ] Core Location
* [ ] Keyboard
* [ ] CFNetwork Framework
* [ ] AVFoundation
* [ ] iTunes Connect
* [ ] App Store
* [ ] Accelerate Framework
* [ ] Accounts Framework
* [ ] App Switcher
* [ ] Audio
* [ ] Audio Toolbox
* [ ] AVKit
* [ ] Calendar
* [ ] CallKit
* [ ] Cellular Service (Calls / Data)
* [ ] CloudKit
* [ ] Contacts
* [ ] Contacts Framework
* [ ] Control Center
* [ ] Core Animation
* [ ] Core Bluetooth
* [ ] Core Data
* [ ] Core Foundation
* [ ] Core Graphics
* [ ] Core Image
* [ ] Core Media
* [ ] Core Motion
* [ ] Core Spotlight
* [ ] Core Telephony Framework
* [ ] Core Video
* [ ] CoreML
* [ ] EventKit
* [ ] External Accessory Framework
* [ ] GameplayKit
* [ ] GLKit
* [ ] HealthKit
* [ ] HomeKit
* [ ] Image I/O
* [ ] Intents Framework
* [ ] IOKit
* [ ] Local Authentication Framework
* [ ] Something not on this list

## Base Information for this issue

1. iOS System Version：iOS 11 beta
2. Language：
* [x] Objective-C
* [x] Swift

3. Prototype（是否是真机）
* [x] YES
* [ ] NO

4. Test devices：

* [ ] iPhone 7

* [ ] iPhone 7 Plus

* [ ] iPhone SE, iPhone 5s

* [x] iPhone 6s, iPhone 6

* [x] iPhone 6s Plus, iPhone 6 Plus

* [ ] iPad Pro (10.5-inch), iPad Pro (12.9-inch) (2nd generation)

* [ ] iPad (9.7-inch), iPad mini 4, iPad Air 2, iPad mini 3

* [ ] iPad Pro (9.7‑inch)

* [ ] iPad Pro (12.9‑inch)

* [ ] iPad Air, iPad mini 2

* [ ] iPod touch (6th generation)

5. What kind of issue are you reporting?
* [x] Q-A
* [ ] Crash
* [x] Bug

## What steps can we take to reproduce this issue?

**Steps to Reproduce：**

使用 xib 创建 tableView 的 header 的时候出现缺少20的距离, 如图:
(白色区域就是添加的 header 明显上边少了20 point)
(效果图)

How to solve ?

## Here is a Demo.

[iOS_11Test.zip](https://github.com/ChenYilong/iOS11AdaptationTips/files/1059867/iOS_11Test.zip)

## Same questions on Stack OverFlow

no same questions